Hi, Kaspersky has been causing trouble for other users, so try disabling it, together with CCleaner which sounds as if it is hooking into Firefox so may well be causing this. On Win10 the default Windows Defender should be all that you need to avoid conflicts, which is the most likely cause of failure to start.

If it starts with security programs disabled, you can see if there are any updates for them.

If the above fails, you could try restarting your PC - click on Restart (don't shutdown then start with the power button, because in Win10, Shutdown is actually a type of hibernation).

You could also try reinstalling Firefox: With Firefox closed, use another browser to download Firefox from https://www.mozilla.org/en-US/firefox/all/ and save the file to somewhere convenient on your computer. You can now double click the file to install Firefox. (This process does not remove your Firefox profile data (such as bookmarks and passwords), since that information is stored in a different location.)

If your question is resolved by this or another answer, please help other users by marking the best reply as Solved. Thank you!

Scribe said

Hi, Kaspersky has been causing trouble for other users, so try disabling it, together with CCleaner which sounds as if it is hooking into Firefox so may well be causing this. On Win10 the default Windows Defender should be all that you need to avoid conflicts, which is the most likely cause of failure to start. If it starts with security programs disabled, you can see if there are any updates for them.

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If the above fails, you could try restarting your PC - click on Restart (don't shutdown then start with the power button, because in Win10, Shutdown is actually a type of hibernation).

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You could also try reinstalling Firefox: With Firefox closed, use another browser to download Firefox from https://www.mozilla.org/en-US/firefox/all/ and save the file to somewhere convenient on your computer. You can now double click the file to install Firefox. (This process does not remove your Firefox profile data (such as bookmarks and passwords), since that information is stored in a different location.)

If your question is resolved by this or another answer, please help other users by marking the best reply as Solved. Thank you!

I cannot uninstall Firefox nor reinstall over old application. Likewise, I cannot uninstall Kaspersky, even after disabling self protection Haven’t found their removal tool which supposedly exists. Also, cannot do a restore or download an ISO file. Have scanned with Malwarebytes and some other A. From Bleepingcomputer, which do not show viruses. I guess killing the system and doing a clean reinstall may be the best bet. A real pain.

Yes since nothing will uninstall, it's sounding more like an issue with the Win update itself, with which I sympathise! However just in case the update has conflicted with Kaspersky or CCleaner, you could try uninstalling with https://www.revouninstaller.com/products/revo-uninstaller-free/ but I would advise against using it for Firefox, since it may remove your Profile as well, which would delete bookmarks etc., and I suspect that Firefox isn't at fault anyway.

I found Malwarebytes was the culprit, after I disabled it from running on startup, firefox ran perfectly. I checked Kasperspky, but it was okay. Be mindful that as soon as Malwarebytes starts running then it will start "blocking" or slowing down programs with no warning.