Elizabeth de Montfort, Lady Montacute and Furnivall (d. 1354), was the mother of William Montacute, earl of Salisbury (1301-44). Here's a post about her, her marriages and her children.
Elizabeth was the daughter of Peter or Piers de Montfort of Beaudesert in Warwickshire, who died not long before 4 March 1287, and Matilda or Maud de la Mare, and had a brother called John de Montfort, their father's heir.  This branch of the de Montforts was only distantly related to the famous Simon de Montfort, earl of Leicester, who was French by birth and ancestry, though Elizabeth's grandfather Peter de Montfort the elder was a staunch supporter of Simon and was killed alongside him at the battle of Evesham in August 1265. Peter de Montfort the elder was the first ever holder of a parliamentary office later known as the Speaker of the House of Commons.
Elizabeth's date of birth isn't known, but her marriage to her first husband Sir William Montacute or Montagu(e) of Somerset (d. 1319) was arranged in June 1292. The grant of the marriage was made to Elizabeth's brother John as their father was dead by then, and the record of it on the Patent Roll confusingly refers to Elizabeth as John's daughter by mistake.  William was the son and heir of Simon Montacute (d. 1316 or 1317), and his date of birth isn't known either and has been estimated as anywhere between 1265 and 1285. He was one of the many men knighted with Edward of Caernarfon on 22 May 1306. Elizabeth and William's second son William Montacute (d. 1344), earl of Salisbury and a close friend of Edward III, was probably born in 1301 (see below). This means that their eldest son John was born in or before 1300, and several of their daughters might also have been born before 1300. It seems unlikely therefore that either Elizabeth or William was born later than the early 1280s, and they might both have been born in the 1270s (1265 seems much too early to me).
Elizabeth de Montfort and William Montacute had a large family, four sons and six daughters; four of their ten offspring entered the Church. In 1348, Elizabeth founded a chantry to pray for the souls of her parents, her two husbands, and her ten Montacute children. She named her four sons in birth order, then her six daughters also in birth order. They were:
- John, first son, born in 1300 at the latest and perhaps in the mid or late 1290s, who married Joan de Verdon (b. 1303) in Edward II's presence at Windsor Castle on 28 April 1317. The young man died in July or August that year, and Edward II paid for his funeral in Lincoln Cathedral.  There is no doubt that John was the eldest son, as his mother named him as the first of her four sons in 1348 even though he had not been knighted by the time of his death and his brother William (d. 1344) was an earl and thus massively outranked him. The fact that his father William the elder (d. 1319) arranged his, rather than his brother the younger William's, marriage to the heiress Joan de Verdon in April 1317 also indicates that he was the eldest son. John's death in July/August 1317, given that he had been married mere months before, surely came as a shock to everyone, and Edward II seems to have been deeply affected by it.
- William, second son, earl of Salisbury. He was certainly born after 3 May 1300, as on 3 May 1321 Edward II called him 'a minor in the king's ward' when he allowed him seisin of part of his inheritance, and before 21 February 1302, as on 21 February 1323 he was granted full possession of his late father's lands as he had proved his age and done homage to the king. Sadly the proof of age, which would give William's exact date of birth, no longer exists, though the royal order to hold it does, and is dated 29 June 1322. It states that William 'says he is of full age' - which would place his date of birth before 29 June 1301, probably not too long before - and that he was born in Cassington ('Carsyngton'), a village in Oxfordshire not far from Oxford. At his father's inquisition post mortem in April/May 1320, William was said rather vaguely to be seventeen or eighteen years old.
Somewhat peculiarly, Edward II's chamber account of 22 April 1326 states that William had permission from the king to travel to London because he was going to be dubbed a knight, and the entry calls him 'a child in the keeping of the king's chamber'. William was certainly no longer a child in April 1326 and must have been twenty-five or almost by then, so I suspect this might be an error for his younger brother Edward. William married Katherine Grandisson (d. 1349) around 1327 and died on 30 January 1344. He was the father of another William Montacute, earl of Salisbury (b. in Donyatt, Somerset on 19 June 1328, d. 1397), Philippa, countess of March (d. 1382), and several others. 
- Simon, third son, bishop of Worcester and Ely, born in 1303 or 1304. He was a student at Oxford by 29 November 1318 when a petition from Edward II to Pope John XXII stated that he had not yet completed his fifteenth year. He appears, as 'Simon de Mountagu', on 26 March 1317, when Edward II called him 'the king's cousin and clerk'.  Simon was appointed bishop of Worcester in late 1333 when he was probably not yet thirty, and bishop of Ely in 1337. He died in June 1345.
- Edward, fourth and youngest son, most probably the godson of Edward II or at least named in his honour (or was perhaps the godson of Edward I, if he was born before July 1307). Edward Montacute accompanied his father in 1318/19 when William the elder was made steward of Gascony, and he had a boat there called La Peronelle which appears in Edward II's accounts. If I'm correct in thinking that the reference in Edward II's chamber account of April 1326 refers to Edward Montacute rather than his brother William, he was knighted in London not long after 22 April 1326, or at least his knighting was planned then. On the other hand, an entry on the Patent Roll in March 1337 implies that he had recently been knighted by Edward III.  He married Edward I's granddaughter Alice of Norfolk in or before August 1338 and, horribly, beat her to death in the early 1350s. Their two young daughters were subsequently sent to live with their paternal grandmother Elizabeth de Montfort.
- Alice Daubeney, the eldest Montacute daughter, who married Sir Ralph Daubeney or Daubeny or Daubenay of Somerset (b. 3 March 1305), the son and heir of Sir Eli(a)s Daubeney, who died when his son was mere weeks old. Alice was the mother of Sir Giles Daubeney (d. 1386), who was born outside England, though I don't know where.  The family name often appears in medieval records in its Latin form, 'de Albiniaco'.
- Mary Cogan, second daughter; the identity of her husband is not known, and I know absolutely nothing about her.
- Elizabeth Montacute, third daughter, prioress of Haliwell or Holywell in Shoreditch, London from 1340 to 1357. A letter from the abbot of Westminster dated 5 November 1334 states that 'Elizabeth de Monte Acuto, a girl of noble birth, had entered the priory of Halywell by London as a nun...she had nothing of her own to provide for her food and clothing...and they [the abbot and convent of Westminster] out of pity for her poverty' granted her an income of 100 shillings a year. 
- Hawise Bavent, fourth daughter; she married Roger Bavent, whose father Roger Bavent Sr was born in Sussex in March 1279 and was one of the men knighted with Edward of Caernarfon and Hawise's father on 22 May 1306. Roger Jr died on 23 April 1355, and his widow Hawise Montacute was still alive on 13 October 1361. They had a son John, twenty years old in June 1357, who died childless, and a daughter Joan, who married Sir John Dauntsey. 
- Maud/Matilda Montacute, fifth daughter, abbess of Barking Abbey from 1341 to 1352. The abbess of Barking outranked all the other English abbesses, and was always of noble birth. According to the Annales Paulini, Maud's brother Simon, bishop of Ely, and sister Elizabeth, prioress of Haliwell, attended her consecration on 29 April 1341. 
- Isabel Montacute, sixth daughter, who succeeded her sister Maud as abbess of Barking in 1352, and held the position until her death in 1358. Isabel was succeeded by Katherine Sutton and then by her niece Maud Montacute the younger, one of Edward Montacute and Alice of Norfolk's daughters.
The Genealogics website and Wikipedia give William Montacute and Elizabeth de Montfort a seventh daughter, Katherine, who married Sir William Carrington. I don't know which primary source(s) confirm(s) that, but it would be deeply odd if she alone was not mentioned in her mother's list of children in 1348, so without seeing primary source evidence I remain to be convinced that she existed. Genealogics also omits John Montacute, who lived into his teens and appears quite a few times on record. The birth order of the four sons of Elizabeth de Montfort and William Montacute (d. 1319) is clear, as is the birth order of the six daughters, but putting them together is basically impossible. John Montacute was the eldest son and Alice Montacute, later Daubeney, was the eldest daughter, but which of them was the eldest sibling, or whether William the second son was older or younger than Alice or older or younger than the second daughter Mary, is anyone's guess. Edward Montacute the youngest son might have been born in c. 1305 or a decade or so later than that.
Elizabeth de Montfort was widowed in the autumn of 1319 when Sir William Montacute died in Gascony, where Edward II had appointed him steward the year before. News of his death reached Chancery in England on 13 November 1319; the writ to hold his inquisition post mortem was issued belatedly on 9 April 1320; and Elizabeth was granted her dower on 23 May 1320.  She married her second husband Sir Thomas Furnivall of Sheffield, the widower of Hugh Despenser the Elder's sister Joan and a landowner in Yorkshire, Nottinghamshire and Derbyshire, sometime before 8 June 1322, when Edward II fined them £200 for marrying without a royal licence.  In 1318, Thomas Furnivall and Joan Despenser's son Thomas Furnivall the younger had married Joan de Verdon, widow of Elizabeth de Montfort and William Montacute's eldest son John (i.e. Elizabeth's former daughter-in-law married her future stepson). On 11 September 1321, Thomas Furnivall the elder had acknowledged that he owed debts to several of Elizabeth's children, his future stepchildren: £300 to Edward Montacute the fourth son, another £300 to Hawise Montacute the fourth daughter, and £40 to Elizabeth Montacute the third daughter.  Thomas died in 1332, and Elizabeth received her dower on 6 June 1332; she did not marry again.  Her third son Simon became a bishop in 1333, her second son William was made earl of Salisbury in 1337, her third daughter Elizabeth became a prioress in 1340, and her fifth daughter Maud became an abbess in 1341.
Elizabeth de Montfort Montacute Furnivall lived long enough to witness the annulment of her grandson William Montacute's (b. 1328) marriage to Joan of Kent, later princess of Wales, in 1349, and the horrible death of her daughter-in-law Alice of Norfolk in the early 1350s following a terrible beating by Elizabeth's youngest son Edward and some of his retainers, after which Elizabeth looked after her toddler granddaughters for a while. She died in August 1354, either on the 6th, the 10th, the 19th, the 26th, the 27th or the 29th; the jurors at her inquisition post mortem gave different dates, though she was certainly dead by 30 August when the writ to hold the IPM was issued. She must have been in her mid-seventies or thereabouts, and her grandson William Montacute, earl of Salisbury, then aged twenty-six, was the heir to her extensive Montacute dower lands. One of her manors was Cassington, where her second son Earl William was born in 1301.  Elizabeth had outlived three of her four sons, John, William and Simon, and several of her six daughters. She was buried in St Frideswide's Priory in Oxford, later Christ Church Cathedral.
1) Calendar of Fine Rolls 1272-1307, p. 235.
2) Calendar of Patent Rolls 1281-92, p. 496.
3) Thomas Stapleton, 'A Brief Summary of the Wardrobe Accounts of the tenth, eleventh, and fourteenth years of King Edward the Second', Archaeologia, 26 (1836), p. 339.
4) Calendar of Close Rolls 1318-23, pp. 287, 629; CFR 1319-27, p. 56; Calendar of Inquisitions Post Mortem 1307-27, no. 238; CIPM 1336-46, no. 700; CIPM 1347-52, nos. 64, 244, 310; SAL MS 122, p. 61.
5) Foedera 1272-1307, pp. 379-80; Calendar of Chancery Warrants 1244-1326, p. 465.
6) CPR 1334-38, p. 401.
7) CIPM 1300-07, no. 324; CPR 1350-54, p. 63.
8) CPR 1334-38, pp. 92-3.
9) CIPM 1300-07, no. 55; CIPM 1352-60, no. 387; CCR 1360-64, p. 38; Complete Peerage, vol. 2, p. 34.
10) Annales Paulini 1307-1340, in Chronicles of the Reigns of Edward I and Edward II, ed. W. Stubbs, vol. 1, p. 370.
11) CFR 1319-27, pp. 8-9, 13; CIPM 1317-27, no. 238; CCR 1318-23, p. 192.
12) CFR 1319-27, p. 133.
13) CCR 1318-23, p. 496.
14) CIPM 1327-36, no. 470; CCR 1330-33, pp. 471-2.
15) CIPM 1352-60, no. 173.