I started using it about a month ago, because I was getting annoyed with some bugs on Safari [1]. That was on El Capitan, as my Mac was old enough to not support Sierra. I have since gotten a new Mac and am on High Sierra. Firefox has been working well enough that I have not checked to see if those Safari bugs are gone.
There are only two things I've found that annoy me on an ongoing basis with Quantum, and two things I wish they handled better.
• The English spelling dictionary that comes with it is terrible. You can add a better spell check dictionary from the add-ons page, and that helps. Even with that there are weird gaps, though. Consider these words: millisecond, microsecond, nanosecond, picosecond, femtosecond, attosecond. It knows them except for picosecond and attosecond. It seems odd to me to have femtosecond but not picosecond.
Chrome knows them all except attosecond. Safari knows them all.
Although it does not know picosecond, it DOES know picoseconds. Huh?
• I miss the "look up" command on the right-click menus from Chrome and Safari to look up a word in the MacOS dictionary and present the results in a pop-up. I can use the system keyboard shortcut cmd-option-d to open a selected word in Dictionary, but I like the pop-up better. I've got an add-on that adds a lookup to the right-click menu, but that just opens a web-based dictionary in a separate tab.
The two things I wished they handled better were certificates and containers/profiles.
• Safari and Chrome use Apple's keychain for certificates. If I want to add a non-standard certificate authority's root certificate, or trust a self-signed certificate, all I have to do is import it via Keychain Access and set the trust level and it then works in Safari and Chrome (and Mail and Messages...). Firefox uses its own certificate management, requiring that I add everything to it separately.
• Using more than one profile is awkward compared to Chrome. If all you care about is keeping cookies separate [2], Firefox container tabs look like a good alternative, but I also want to keep bookmarks, caches, and history separate, which seems to be beyond what container tabs can handle.
[1] There were two that were particularly annoying. (1) When returning to a page via the back button (or corresponding gesture) it would sometimes freeze for a few seconds after it displayed the page. (2) Occasionally it would get in a state where during a back gesture it would freeze in the middle of the animation of the previous page sliding in. I never saw it recover from that.
[2] Keeping cookies separate is probably good enough for a large fraction of the things that would drive people to multiple profiles. It suffices for things like checking a work account from home or a home account from work on a service where you have accounts for both home and work.
