I've had some extremely weird hangs in Firefox particularly around when I unlock my (Windows 10) PC. It happens extremely consistently even in troubleshoot mode and after several reinstalls of Firefox. Occasionally it also seems to happen when an external application asks the operating system to open a URL (Since Firefox is my default browser this of course causes Firefox to open a new tab... eventually). The exact amount of time the hang lasts seems to vary quite a bit, but it's usually less than 30 seconds. Even when the hang lasts a long time I have never seen Windows indicate that the app is not responding which seems to imply it's not getting stuck so much as getting slowed down by something. I can reproduce the unlocking version of this issue extremely reliably. In Task Manager I can see all of the Firefox processes sitting at 0% CPU, but when I lock and unlock the computer (with Windows Key + L to lock) exactly one of the Firefox processes (the ones grouped together in task manager) shoots up to ~8% CPU for the duration of the hang. The process that reliably spikes is using~500MB of memory while all of the other Firefox processes average about 40MB of memory so I assume the spike is happening in the main Firefox process. One of the two Firefox processes that ISN'T grouped together with any others in Task Manager also spikes to ~4% CPU when the hang starts fairly reliably, and some of the other grouped processes usually spike but to a much lesser degree and for a much shorter time.