do you have other users on the PC, login as one of them and see if it happens, or create another firefox profile and try that.
ctrl-shift-del to delete all temp files?
you could try the developer edition
https://www.mozilla.org/en-GB/firefox/developer/
or waterfox,
https://www.waterfox.net/
or palemoon
http://www.palemoon.org/
to see if they have same problem, they're based on firefox