hi, does it help if you download the full installer from https://www.mozilla.org/en-US/firefox/all/ and run the setup again?

Will I lose my bookmarks when I reinstall Firefox or is there a way to save the bookmarks?

The settings are not in the Firefox program folder but in a separate place. You can backup your Profiles just in case though.

https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data

An error message about XPCOM missing or about platform version usually happens because of a failed Firefox update that left you with a mixture of old and updated files in the Firefox program 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 can do a clean reinstall to ensure that all files are replaced. You may have to disable security software temporarily in case the problem persists.