I had a similar issue after I upgraded where I got an error message stating that Firefox was already open, and I needed to close before I could open a new browser. Booted in safe mode and still had the issue. I ended having to uninstall (delete files option), delete the registry keys under HKLM/software/mozilla/firefox. I deleted every key that mentioned firefox. Restarted pc, reinstalled firefox, updated then imported my bookmarks. ( Forgot to mention I copied my profile to a USB stick before the uninstall) Everything is fine now, but when it was not responding correctly at the beginning I started having doubts about my beloved Firefox. This may or may not help but I went through this process just last night.
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