Hi Roman, is it the page for one specific extension, or Firefox's built-in Add-ons page?

Does it also appear in new windows (Ctrl+n)?

If it's Firefox's page, check your home page setting to make sure about:addons did not get added somehow. More info in this article: How to set the home page.

Cntrl+n shows the correct page but when I restart the browser this link always appears:

https://addons.mozilla.org/en-US/firefox/

Hmm, that's so strange.

On Windows, I would ask you to check whether your desktop shortcut's command line had that URL added to it. I don't know whether that is a thing on Linux.

Or is Firefox trying to alert you to some add-on or theme-related problem???

I think the problem exists when I updated from Firefox 63esr to Firefox 78esr.

What if I deleted "pref.js" located in my profile directory? That should delete any strange preferences.

I can try reinstalling. Yes?

Firefox tries to suggest a different addon each time Firefox starts up.

The prefs.js file stores the custom settings you see in about:config. You could rename it temporarily as a test -- while Firefox closed, of course (prefsOLD.js).

That did the trick! Renaming prefs.js to prefsOLD.js, restarting and selecting a new webpage from the bookmarks now works.

Thank you for your time in helping me resolve this problem.

Roman