There's no need to remove, just install it again with a full installer.

The full offline setup for Windows can be downloaded from https://www.mozilla.org/firefox/all/#product-desktop-release as downloading Firefox from Windows from mozilla.org may serve you the small online stub installer.

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 reinstall Firefox and possibly 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.

I had a case were reinstallation did make Firefox work again, but lost all bookmarks and so one. The solution was close Firefox, and going to the user account at:

C:\Users\username\AppData\Roaming\Mozilla\Firefox\Profiles

where there was the old profile named xab79nof.default and a new empty one 8bi9noyc.default-1642063237913 so renamed them:

8bi9noyc.default-1642063237913 -> 8bi9noyc.default-1642063237913.off xab79nof.default to 8bi9noyc.default-1642063237913

when Firefox started, there was all in place; those bookmarks were quite important to the user.

Sometimes a new profile is created by error. To fix it, it's enough to enter about:profiles in the address bar and select your old profile. You don't need rename any files.

It is not a good idea to rename profile folders on the hard drive as this can cause issues as several files have absolute file paths. If a profile isn't registered (i.w. doesn't show in the Profile Manager) then you should register this profile by using "Choose Folder".

The solution to reinstall is definitely not helpful. As others have pointed out, all customisations, including stored passwords are lost, and reinstating those by the "forgotten password" routine for every site ever used is mighty inconvenient. Any way out of the problem on my wife's computer would be good. As I have been putting my PC to sleep rather than shutting down, my "old" version is still working. Is there a way to kill the downloaded update which has yet to be installed, and do a manual update from elsewhere that doesn't involve total loss of essential information which, incidentally, has been the fundamental problem with Firefox from the beginning, be it due to errors, crashes, or just updating to a new computer.

@Chris, did you try to switch profiles?

No. Unfortunately my preserved old version which worked when I posted earlier now aborts all new tabs including those opened by firefox it's self, like this help page, with a message that the tab won't load, cancel or reload. Reload produces the same error repeatedly. So I went for troubleshoot, and now have my wife's problem, version "127.0.1" is now compatible ...... So now I can't install a new version as I have no web browser! Catch 22.

This specific message about platform version 127.0.1 not compatible is about the Firefox installation folder and has nothing to do with the profile folder that stores your personal data. There is no need to remove your Firefox profile during the reinstall and Firefox should pickup your personal data after a successfully reinstall, so you shouldn't lose anything although your software might still cause issues since that is the most likely cause of this problem.

This just happened to me, I took the advice and downloaded firefox web browzer, once it downloaded the firefox installer.exe application I then clicked on it, it then downloaded the app which took about 1 minute and then automatically started and looked exactly like before the error, meaning all my bookmarks etc worked. Thank you to the poster who replied with the solution.