I have tried to download and install classic theme restorer and tabs on bottom. Both download, but they do not install. They do not show up on my downloads list nor my add on list.
I suggest you look for help on our support site: https://support.mozilla.org/
Have you noticed any error messages, maybe in the Browser Console?
Your support page, is difficult to navigate when looking for a specific answer. I was unable to find anything related to my problem. There were no error messages.
You might find your answer here: https://support.mozilla.org/t5/Problems-with-add-ons-plugins-or/Unable-to-install-add-ons-extensions-or-themes/ta-p/1258