I switched to using ublock origin to do that, since I can quickly add an exception for a site. Some sites use fonts for icons. Most of them are still usable but just look a bit silly, but sometimes it's annoying to the point that I want to add an exception.

Let's see if I get this right:

(0. Install uBlock Origin)

1. Re-enable web fonts

2. In the main UO settings, check "Block remote fonts"?

3. Refresh a page

4. Click the UO icon, and then "more" until I see an icon titled "Click to no longer block remote fonts on this site"

5. Click

6. Refresh the page

Repeat 4-6 for relevant sites.

Any caveats?

Yep that's how I so that, although in step 4 I don't have to click "more", I immediately have an icon with a capital A that toggles fonts at the bottom. But maybe I configured it to automatically expand something that is by default hidden behind a more button... I honestly don't remember.

Oh, also, I have to remember to click the "Save" button, otherwise it resets when I restart Firefox.