(Old review Title field that was silently removed per typical Mozilla doctrine of "rip away longstanding useful features Because Modernization":)
== Great at font inspection, but the claimed font-adjusting capability seems to be missing (not so on XUL version) ==

Instead of the "4 key pieces of functionality" listed for the old XUL version of this add-on, this WebExtension version's description lists "3 key pieces". Unfortunately, as far as I can tell, only the first two have actually been implemented. Supposedly, "Any active element can have any piece of the font's options (such as color, size or family) adjusted inline", but with the WebExtension version, the context menu only has a single option, "Inspect Font", and none of the fields in its popup are modifiable. I also checked the screenshots and the FAQ (which, BTW, uses the misspelling "Fond Finder" in multiple places), to no avail.

If you need the actual font substitution functionality, it appears that for now you'll need to stick with the XUL version of Font Finder (which of course will stop working next month unless you migrate to Firefox ESR or a fork like Pale Moon). Just watch out for the infinite popup loop the XUL version gives you the first time you restart after installing it in a profile — see my review of that add-on for more details.

My final note is that neither the XUL version nor the WebExtension version actually address the problem I installed them for. Their descriptions say "Information Captured: [...] Font family (including actual font being rendered!)", which I was expecting to help me debug the problem I was having on Linux with extremely messed-up scaled-bitmap, all-caps font substitutions (even when the exact TrueType font specified in the stylesheet was installed). Unfortunately they don't tell you the "actual font being rendered" — they just tell you the font that the web page *requests* to be rendered. I then learned about running Firefox with the environment variable FC_DEBUG=1, but its output didn't give me any useful information about the incorrect substitution. Luckily after a bunch more Googling, I was able to figure out that going to about:config and setting gfx.font_rendering.fontconfig.fontlist.enabled = false would cause Linux Firefox to go back to the old Cairo-based font rendering rather than the broken Firefox 44+ version.