An error message like XPCOM missing or about platform version or entry points not found can happen because of a failed Firefox update that left you with a mixture of old and updated files in the Firefox installation folder. This can happen in case security software was keeping a handle of some of the files in the Firefox program folder preventing Firefox from replacing all the files. You need to do a clean reinstall and remove the Firefox program folder to ensure that all files are replaced. You may have to disable security software temporarily in case the problem persists.

• https://www.mozilla.org/en-US/firefox/all/#product-desktop-release

I still can't get my original Firefox back. When you state "You need to do a clean reinstall and remove the Firefox program folder to ensure that all files are replaced" do I download Firefox again and how do I remove the Firefox program folder to ensure that all files are replaced.

There are three steps:

(1) Download the full installer from

https://www.mozilla.org/en-US/firefox/all/#product-desktop-release

to a convenient location.

(2) Delete the program folder which usually is here:

C:\Program Files\Mozilla Firefox

To confirm, you can check the Properties for your Firefox desktop shortcut: right-click > Properties, then on the Shortcut tab, the Target line shows where the program is running from.

(3) Run the installer you downloaded in Step #1. It should detect your existing Firefox data and launch with your bookmarks, saved passwords, etc.

Success?

I still need help. I've followed the steps above, ran the installer but Firefox data was not detected and didn't launch bookmarks and passwords. Does "Delete the program folder on C:\Program Files\Mozilla Firefox" mean uninstall, which I've done.

No, uninstall was not needed. When you run the uninstaller, you may have the option to keep or erase your data. Hopefully you did not erase your data.

Could you check the first section of the following article about checking for available profiles:

Recover lost or missing Bookmarks