Firefox add-ons not (all) working

Today I had a funny thing happening with Firefox. Some Add-ons (Extensions) appeared to be not working, but the one which I use via a keyboard shortcut did work. The Add-ons dialog (via Tools/Add-Ons) also didn't work, all the screens didn't show the installed add-ons nor did it show the add-ons site.

The solution is described at the Mozilla support forum, but not very clearly, so here's the minimum amount of steps:

1. On the menu bar, click on the Help menu and select Troubleshooting Information. The Troubleshooting Information tab will open.
2. Under the Application Basics section, click on Show in Finder (on Windows it might be called Explorer, I don't know, don't use it anymore). A window with your profile folder will open.
3. Go back to Firefox and quit it completely
4. Go to the folder window opened in step 2
5. Delete all files with the name extensions (extensions.rdf, extensions.cache, extensions.ini, extensions.sqlite) and compatibility.ini in the Firefox profile folder. Do NOT delete the folder called extensions!
6. Start Firefox again. All add-ons will now behave as being newly installed, so Firefox might have to restart itself again)