Contributed 3rd Party Builds • Re: Pale Moon x86-64 SSE2/AVX2

but my question was whether it would crash the whole browser or just the player in its respective tab where the video was attempted open.

The application would crash (on any reasonable OS anyway). If a CPU encounters an illegal instruction for its hardware then there is no sensible way to continue execution in that process, and the only option is to exit with an error. Unless video playback would be handled entirely externally (e.g. using an out-of-process plugin) and in a fault-tolerant manner, there's no way the browser can recover and continue execution.