I'm not sure what can cause this issue and I don't want to start a panic but there's a small possibility this could be a keylogger or another virus. The fact that both processes use similar amounts of memory make it somewhat unlikely but just to be sure I'd give my PC a quick scan with both anti-malware software (this one) and one with an antivirus program of your choice (windows defender is fine). Like you said it could simply be a result of a recent update. Other than that - well, I gave up on firefox a long time ago since it became increasingly unreliable. Chrome has its own set of problems but it works 99% of the time which is kinda important.

I uninstalled latest update and installed the previous one, and now there is only one firefox.exe showing up and its stop crashing. Kinda funny I thought the reason for there being more than one instance was to improve stability.

I had a problem with FireFox that it was running very slow and eating up my CPU, tried a bunch of things and just got annoyed so switched over to Chorme. You can import all your bookmarks to it, quite like it now.

I had a problem with FireFox that it was running very slow and eating up my CPU, tried a bunch of things and just got annoyed so switched over to Chorme. You can import all your bookmarks to it, quite like it now.