So you are saying you cannot install the Firefox 135.0.1 package from Mint http://packages.linuxmint.com/search.php?release=xia&section=any&keyword=firefox

The Firefox tarball from https://www.mozilla.org/firefox/all/desktop-release/ is basically just an archive as there is no package or installer or such. You just extract and run the firefox-bin to start Firefox is the basics. You can then create a launcher shortcut to the firefox-bin and use a Firefox icon from the /firefox/browser/chrome/icons/default/ folder. Btw the mention of chrome in this case has nothing to do with Google Chrome. Oh and make sure the firefox folder has read/write permissions for the user to get internal Firefox updates from Mozilla as a easy way if you are the only user is to have the firefox folder in /home

I'm sorry, but this was not my question.

Yes, I can install Firefox from LinuxMint-Repository (LinuxMint package manager) - no problem. This installs the LinuxMint version of Firefox. But I will avoid this installation option (this Firefox version!).

Instead of installing Firefox from LinuxMint as repository source, I want to add Mozilla release repository as an alternative package source in LinuxMint package manager. When entering the command "[sudo] apt-get install firefox", LinuxMint should use the Mozilla source for downloading, installung and later updating Firefox. I share the first installation option, the authors describs it their article. Therefore I followed their install instructions step by step.

My problem is: When entering the command "[sudo] apt-get install firefox", the LinuxMint version of Firefox will be installed allways. (But) The behaviour should be: checking, downlading and installing/updating Firefox from Mozilla release repository instead.

Your installation suggestion is one of the last suggestions in that article.