Sorry to hear about this problem.

Sometimes program installers and uninstallers mess with certain shared Microsoft files, often triggering a message related to a missing VCRUNTIME140_1.DLL file. MIssing that DLL can cripple Firefox.

To rule out that issue, you can (re)run Microsoft's installer for the VC Runtime files:

(A) Open https://docs.microsoft.com/en-US/cpp/windows/latest-supported-vc-redist?view=msvc-160#visual-studio-2015-2017-2019-and-2022

(B) In the "Visual Studio 2015, 2017, 2019, and 2022" section of the article, download the installer for your system:

• 64-bit Windows: the "X64" download
• 32-bit Windows: the "X86" download

(C) Run that installer and restart Windows.

Any improvement?

If the above does not help, Some problems occur when your Internet security program was set to trust the previous version of Firefox, but no longer recognizes your updated version as trusted. Now how to fix the problem: To allow Firefox to connect to the Internet again;

• Make sure your Internet security software is up-to-date (i.e. you are running the latest version).
• Remove Firefox from your program's list of trusted or recognized programs. For detailed instructions, see

Configure firewalls so that Firefox can access the Internet. {web link}

Neither helped so far.

VC Runtime ran and restarted, no effect.

I have WebRoot and Windows defender. Removed it in both, re-added - no effect... Completely turned off both, nothing there, either.

Now firefox will barely run. 1/10 times it will open the blank new tab, unable to load anything. The other times, it shows up in Task Manager, but not visibly.

Got it! WebRoot WAS the issue, however it required a reinstall of that program. Common issue from them, apparently.

That was very good work. Well Done.

https://www.mozilla.org/firefox/97.0.1/releasenotes/

Fixed Works around problems with WebRoot SecureAnywhere antivirus rendering Firefox unusable in some situations