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 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.

You can find the full version of the current Firefox release (91.0.2) in all languages and all operating systems here:

• https://www.mozilla.org/en-US/firefox/all/#product-desktop-release
• https://www.mozilla.org/en-US/firefox/91.0.2/releasenotes/

So, I interrupted the updating process. Is there a download link just for the update to 91.0.2, so I don't have to re-install my extensions?

You can download the full version via the link I posted above.

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

The profile folder that stores your personal data is stored elsewhere, so you won't lose bookmarks and extensions.

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