Sorry to hear about the crashes. Did you get the Mozilla Crash Reporter dialog? If so, your Firefox may have logged some data that would help track down the cause of the problem. You can submit that data to Mozilla and share it with forum volunteers to see whether it points to the solution. Please check the last section of the support article "Firefox Crashes" for steps to get those crash IDs from the about:crashes page, and then post some of the recent ones here. Thanks.

Also, if you use Websense security software, please see this thread: https://support.mozilla.org/questions/1078511

crelfe said

Have tried a clean install on a number of Windows 7 Enterprise computers. All Firefox instances 40.0.2 crash on opening the program. Only fix is to back rev to version 39 and tell it not to auto update.

Found out our problem is with Websense Triton. It causes Firefox to crash.

crelfe, the problem is indeed Websense Endpoint. The only thing I can suggest is that you contact them. Their official statement is that Firefox 40 is not supported yet. But causing a program to crash is something else, especially because adding Firefox to the Endpoint's whitelist does not help.

See https://bugzilla.mozilla.org/show_bug.cgi?id=1181091 for details.

In case you run 64 bit Windows, a workaround might be to install a 64 bit build of Firefox. It does not crash in my situation.