I read in the FAQ that the Tor project officially doesn't recommend further Firefox add-ons. But it so happens that I won't use the Tor browser unless I get Adblock Edge to work like in my regular browser.
So how can I install it? I am on Linux. I open the Add-ons, select Adblock Edge 18.104.22.168-signed, click 'install'. It downloads something then responds:
There was an error downloading Adblock Edge.