Please consider turning on send info to Mozilla before posting a question. This helps us help you. As I have no info from your system that would help see things that cause this issue/s.

The crash report is a shutdown hang where Firefox takes too long to close. This is often related to outdated and or corrupted video card drivers and Flash.

https://support.mozilla.org/en-US/kb/upgrade-graphics-drivers-use-hardware-acceleration TRY NO ACCELERATION and https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems?redirectlocale=en-US&redirectslug=Troubleshooting+extensions+and+themes

Please let us know if this solved your issue or if need further assistance.

Do you have any other recent crash reports that might help show a pattern?

This one indicates Firefox was in its Safe Mode -- all extensions disabled, hardware acceleration disabled, JavaScript compilers disabled -- so that rules out a lot of possibilities.

The report does indicate memory was 80% full, and possibly some of the pre-crash functions were seeking to access more memory, but the report does not say Firefox crashed due to running out of memory. I'm not sure what to make of it (I'm not a software engineer!).