This vid contains tips that help fix Class not registered while trying to view some local/network file, or open some app (e.g., TeamSpeak 3) you installed.
Solution 1: Run System File Checker
sfc /scannow (note the space between sfc and "/")
If SFC did not find any violations it is still a good idea to run DISM Restorehealth
Dism /Online /Cleanup-Image /CheckHealth
If you have trouble completing the cmd codes above, perform a clean boot in Windows.
Try reinstalling/repairing your Firefox -> if that does not work, completely remove it using the recommended uninstall tool in this video guide > restart > perform a clean install of Firefox.