Install Firefox again with a full installer. You shouldn't lose any data.

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.

What security software do you have as I've recently seen a rather large amount of this issue?