I used it for a few years, but sometime around the end of 2021 I switched to Chrome because I got tired of the Firefox spell checker marking correctly spelled words as incorrectly spelled.
Here's a list of words I reported in 2020 and 2021 on the open issue they keep in their bug tracker for this problem:
ad hominem, algorithmically, all-nighter, another's, auditable, automata, backlight, ballistically, blacksmithing, bubonic, cantina, chewable, coaxially, commenter, conferenced, counterintuitive, dominator, epicycle, ethicist, exonerations, ferrite, fineable, hatchling, impaction, implementer, implementor, inductor, initializer, intercellular, irrevocability, licensor, lifecycle, manticore, massless, measurer, meerkats, micropayments, mischaracterization, misclassification, misclassified, mistyped, mosquitos, partygoers, passthrough, per se, phosphine, plough, pre-programmed, preprogrammed, programmability, prosecutable, recertification, responder, retransmission, rotator, seatbelt, sensationalistic, shapeshifting, solvability, spectrogram, splitter, subparagraphs, subtractive, surveil, survivorship, synchronizer, tradable, tradeoffs, transactional, trichotomy, tunable, underspecified, untraceably, untyped, verifiability, verifier, webmail
It was kind of puzzling because Firefox actually uses the same open source spell check engine that Google, Apple, LibreOffice, and many others use, and all those others had no trouble with any of the words that Firefox got wrong. This suggests that it might just be a dictionary problem and Firefox could simply grab the dictionary that say LibreOffice uses.
I mentioned that on one of my comments in the bug tracker and a developer said they were going to look into it but I don't know if anything ever came from that. All the words I reported now work but I don't know if they just added those to their dictionary or switched to a better dictionary.
Maybe I'll try doing my HN commenting from Firefox for a while and see if is fixed.
hominem, implementor, plough, passthrough, seatbelt, underspecified, untraceably, untyped
The rest are all fine. (Plough is especially weird!)
I used to get teased (all in good fun) at my last company for being the only dev that used Firefox. Then I'd be the only one to ever catch the Firefox only bugs in PRs.
I get it - it's got way less market share, and for a small company, you may be able to argue the value of the time vs the probability of a bug, but not having your website work on all browsers in the 2020s (aside from new browser features like WebGPU and file system access) is a let down.