(Update: I noticed that noscript still wasn't working on the Tor Browser after having the hotfix installed. So here's a temporary workaround, don't install any other extensions while the signature requirement is set to "false".)
Yesterday morning, Vera noticed that her browser was suddenly showing her ads.
I checked it out, and it had disabled all the security extensions I had installed on it. I had to switch her to Chromium for the time being, and started to look for answers.
It seems that Mozilla done goofed up there, and the problem was a serious one. They'd made some certificate policy changes to how Firefox should handle extensions, and bricked the whole browser. Not just Firefox, but all the browsers based on Firefox.
I soon learned, that even Tor Browser wasn't unaffected by this, as it suddenly let javascript and ads through. (This is a major problem if any!)
Ok, I left the problem rest for the night, and finally found the answer this morning after some web searching.
First, Mozilla wanted people to enable the "Studies" functionality on Firefox to automatically apply the fix. I soon found out that it wouldn't work on the Tor Browser, as the functionality was shielded and I couldn't set it up. (Probably better, as it would've opened a security hole – however temporary, but still a hole – on my system.)
From Github, NixOS issues of all places, I found the answer to how to apply the fix directly.
So, on this page, search for "name": "Hotfix: Update XPI signing intermediate [Bug 1548973]", and click or copy the address on the "addonUrl"-line.
There was some weirdness in how Tor Browser didn't at first seem to get the fix, but then after I deleted the tor-browser directory and installed it (uncompressed the tar) again, it suddenly started working with the fix applied.
Hope it works for you as it did for me.