Maybe because a few years ago it could be very annoying? It was mostly pretty good at rendering web pages but it had many UI problems that could really get on your nerves after a while.
For example somewhere around late 2020 or early 2021 after several years of using it as my main browser on my Mac I switched because a couple of those problems finally just got too annoying to me.
The main one I remember was that I was posting a fair bit on HN and Reddit and Firefox's spell checker had an extraordinarily high false positive rate.
This was quite baffling, actually, because Firefox uses Hunspell which is the same open source spell checker that LibreOffice, Chrome, MacOS, and many other free and commercial products, and it works great in those with a very low false positive rate.
Here's the ones I hit and reported: 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, tradeoffs, transactional, trichotomy, tunable, underspecified, untraceably, untyped, verifiability, verifier, webmail.