Uninstall your add-ons and plugins for Firefox. Once may be corrupted or incompatible.
Restart machine.
Try again with FF. Update if necessary. Run with no plugins/add ons for a day or so before putting them back.
If problem is not with FF directly then you may have a bad dll driver file that is causing issue.
You might try setting your startup to load your machine with VERY minimal drivers (do not load with printer, scanner, photo, email, adobe, etc).
A lighter startup might avoid the problem.
If you can run clean without them, then begin adding each back one by one.
Possible one of them is causing issues in background and needs to be reinstalled.
(note...I have intermittent internet again. Last night none at all. Today sometimes one page loads then dies... ISP repair team scheduled for TUESDAY!
)