After a quick google, here's two almost identical answers:

https://support.mozilla.org/en-US/questions/1060581

https://support.mozilla.org/en-US/kb...in-has-crashed

Firefox can also have problems with 3rd party Add-ons conflicting with other tools and add-ons. FireFox is also considerably top-heavy these days.

I still recommend using Google Chrome for watching any kind of online media instead of FireFox. Only down-side of Chrome is the option settings are all over the place & some of the tools on screen are at opposite ends to most browsers which can be difficult to get used to.