One thing you may wish to try is to search directly for extensions from the Extensions panel rather than from ‘Get Add-ons’. I found this works better for some reason. (This also cuts down on the all the extraneous add-on information one must navigate on Mozilla’s site.) To be clear: From the ‘Open menu’ stack icon, select Add-ons->Extensions and use the ‘Search all add-ons’ box in the upper right.
I installed uBlock Origin and uMatrix this way, in addition to a few other privacy add-ons.
Viel Glück!