Do you see DLL modules listed here:

• https://support.mozilla.org/en-US/kb/identify-problems-third-party-modules-firefox-windows

yes, two listed

{

 "modules": [
   {
     "name": "nvwgf2umx.dll",
     "fileVersion": "31.0.15.2802",
     "typeFlags": 1,
     "signedBy": "NVIDIA Corporation-PE-Prod-Sha1",
     "events": [
       {
         "processType": "browser",
         "processID": 18296,
         "threadID": 3808,
         "loadStatus": 0,
         "loadDurationMS": 1.5321
       },
       {
         "processType": "browser",
         "processID": 18296,
         "threadID": 3808,
         "loadStatus": 0,
         "loadDurationMS": 1.4564
       },
       {
         "processType": "rdd",
         "processID": 2396,
         "threadID": 16612,
         "loadStatus": 0,
         "loadDurationMS": 2.2519
       }
     ]
   },
   {
     "name": "nvldumdx.dll",
     "fileVersion": "31.0.15.2802",
     "typeFlags": 1,
     "signedBy": "NVIDIA Corporation-PE-Prod-Sha1",
     "events": [
       {
         "processType": "browser",
         "processID": 18296,
         "threadID": 3808,
         "loadStatus": 0,
         "loadDurationMS": 0.2476
       },
       {
         "processType": "browser",
         "processID": 18296,
         "threadID": 3808,
         "loadStatus": 0,
         "loadDurationMS": 0.2596
       },
       {
         "processType": "rdd",
         "processID": 2396,
         "threadID": 16612,
         "loadStatus": 0,
         "loadDurationMS": 0.2639
       }
     ]
   }
 ],
 "blocked": []

}

This happens on occasion to me too and it's very predictable. Sometimes after visiting Hotmail (login.live.com) then signing out memory in one process does not go down but keeps going up. Eventually when it reaches over 1,000,000kbs in Task Manager the browser crashes.

So I keep an eye on it and when a process reaches that level I end that one only in TM and the browser recovers. There might be other sites that cause similar problems but for me Hotmail is the only culprit found so far.

I've been considering filing a bug with Mozilla but the problem is it doesn't happen every time so it might be hard for them to reproduce.