Separate Issue; Your System Details shows;

Installed Plug-ins

Shockwave Flash 18.0 r0 Shockwave Flash 11.9 r900
Having more than one version of a program may cause issues. You must remove the older programs. Then get the current full installer.

Flash: Grab the uninstaller from here: Uninstall Flash Player | Windows Uninstall Flash Player | Mac

Then reinstall the latest version.

Flash Player Version 18.0.0.232
https://www.adobe.com/products/flashplayer/distribution3.html Note: Windows users must download the “Internet Explorer” and “Plugin-based browsers” installers.

The crash report flagged these programs;

Signature shutdownhang

atidxx32.dll = Advanced Micro Devices = http://support.amd.com/en-us/download/desktop?os=Windows+7+-+64

This is for Sumo's Related Bugs

1161090 NEW --- crash in shutdownhang | WaitForSingleObjectEx | WaitForSingleObject | PR_Wait | nsThread::ProcessNextEvent(bool, bool*) | NS_ProcessNextEvent(nsIThread*, bool) | mozilla::dom::workers::RuntimeService::Cleanup()

Had the same problems with 40.0.2, and I tried EVERYTHING including closing open tabs, safe mode, new profile, refresh, and reinstalling (so it WASN'T an issue with Shockwave Flash)!

The ONLY and I mean ONLY thing that worked (after hours of wasted troubleshooting) was downgrading to 39.0.3, and haven't had an issue since!

so a lot of people are "hearting" this reply.

so after being a user from the earliest netscape the fix is use an earlier version and refuse all updates?

guess I'll look into it.

Plus look, again, at how I might make chrome meet my needs.

I had a similar problem with firefox 40 on Windows 7. I had the impression that the crashes occured on sites which had a lot flash on them (maybe two flash-plugins overlapping). When switching to later versions of firefox (I don't remember if it was 41 or some 40.0.?-release) the crashes went away, but I got very frequent messages that some script is busy when looking at pages which had a lot of flash.

After that I switched back to firefox 39 like described in one of the answers to this question and it worked again. Then I looked at the change log of version 40 and found that the vsync feature was new. I switched off the three configurations gfx.vsync.* and now I can run firefox 41.0.2 without any crashes.