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.

I believe your Firefox was unable to update successfully and since not all of the files got upgraded, there's a mismatch between the old files & the new files causing that error.

Do you use a security software called ZoneAlarm or ZoneAlarm Extreme Security? It may be causing the problem. If you can find the option to "Clear Virtual Data" that should allow you to reinstall or update Firefox to fix that error message.

Instructions for "Clear Virtual Data" - should be in the Advanced section of ZoneAlarm's settings: http://download.zonealarm.com/bin/forcefield_x/help/wwhelp/wwhimpl/common/html/wwhelp.htm?context=ForceField&file=advancedsettings.html

If you don't use any Zonealarm products then we need to know exactly what security software you use on your computer. So far no one with this exact error has told us what they are using & without that information, you may be seeing this error forever since we can not contact the other company to find out what they are doing to break Firefox.

So please tell us what security software or sandbox software you are using. When this error use to appear over 10 years ago (yes, that's how rare this is), it was caused by security & sandbox software like Zonealarm Extreme security & Sandboxie.