Hi fastcat, that message usually indicates a problem in Firefox's program folder. Could you try:

Clean Reinstall

This takes about 5 minutes. It's not about your settings, it's about making sure the program files are clean (no inconsistent or alien code files). As described below, this process does not disturb your existing settings. It's not essential to uninstall Firefox, but you can if you like, saying No to any request about removing personal data.

It only takes a few minutes.

(A) Download a fresh installer for Firefox to a convenient location:

https://www.mozilla.org/firefox/all/

(B) Exit out of Firefox (if applicable).

If you use Microsoft Office, please change your default browser to Internet Explorer temporarily before the next step.

(C) Using Windows Explorer/My Computer (hold down the Windows key and press E to launch it), right-click > rename the program folder as follows (you might have one or both):

C:\Program Files (x86)\Mozilla Firefox =to=> C:\Program Files (x86)\OldFirefox

C:\Program Files\Mozilla Firefox =to=> C:\Program Files\OldFirefox

(D) Run the installer you downloaded in step (A). It should automatically connect to your existing settings.

Any improvement?

Didn't work. Same XPCOM error response. Did exactly as you had described.

If you didn't already, could you also shut down Windows 10 and restart it, then test again.

If problems continue, could you make sure you are not trying to run Firefox as Administrator, just as your regular user?

I did a complete uninstall, clean reg, restart and re-install on another drive partition, made sure it wasn't admin and I still get the XPCOM error.

Hmm, this is strange.

Can I assume your security software isn't quarantining or deleting any Firefox files after installation?

Could you test whether this makes any difference:

In the Firefox program folder, for example,

C:\Program Files (x86)\Mozilla Firefox

right-click firefox.exe and Rename it to firefox1.exe

Then double-click it to launch it. Any difference?

Also, in case this is all caused by malware, please run some of the cleaners listed in this support article: Troubleshoot Firefox issues caused by malware.

Changing it to firefox1 give same XPCOM error. Interestingly enough, I tried opening my Malware software and it had a fatal error. Uninstalled Malwarebytes and re-installed. Now I get application was unable to start correctly 0xc0000022 error for mbam.exe Virus maybe?

It is sounding suspicious. You could check out the specialized forums in the malware article and they can walk you through some deep cleaning. (It's beyond our scope here.)

Thanks for your help. I’ll have to look at other causes.