Seems, that I finally got it managed -). The way seems a bit weird, but at least I got it.
Went to the addons page and searched for the plugin. There it says "Install Firefox" and I clicked that. That next page said, "You are using the latest version". There is also a button "cleanup firefox". I did, and lost all settings, add-ons and so on.
After closing and re-opening the browser, it looked like a "virgin" one -). Went to the addons page and....a miracle...now It was possible to install the emsisoft plugin and all others (again).
Thanks to anyone who tried to help.