a bad add-on.
We generally say in these forums that "Internet Explorer" problems can be traced back to bad or corrupted add-ons, but rarely do we say that for Firefox.
And in fact I am surprised Firefox is listed in your Task Manager Processes tab after a reboot. Is that a reboot of the whole OS, or just a close down of FF and an attempted restart of it? I may have got that wrong.
If it is listed in Processes after a reboot of the OS, then go hunting for what is attempting to load FF on Startup. You can use MSCONFIG for that, (type that into the Run command in the Start Menu). If you find any FF entries in the Startup tab for FF, disable them.
But if it is not that, then back to add-ons. Try restarting FF with no add-ons, (Firefox button > Help > Restart Firefox with add-ons disabled), and test it.