Firefox is pretty good. The only things I miss are small UI things like muting a website rather than a tab, spell checking, and being able to control the privacy settings for a site directly from a tab rather than having to go into settings and inserting a rule by typing the domain.
Also video playback can be less smooth occasionally.
Firefox has spell checking [0] built-in. Granted, if you want to use several languages it's possible, but clunky - you have to change it every time.
There is also the Language Tool extension [1]. I've discovered this recently and I'm really impressed. I don't use the online service, I run my own server with French and English n-grams and the experience is very good. It auto-detects the language and integrates well in the UI.
There is a tool for merging dictionaries (these are aspell dictionaries internally, iirc), and a number of ready-made merged dictionaries, like English + German, English + Russian, etc. I can attest they work well, without the need to switch.
I wish transparent multi-language support was built in, though.
I am litterally typing "happy Birtdhay to my sister!" on an english version of firefox and see no red underlining ("check your spelling as you type" is checked in the settings).
The feature must be broken or blocked by adblockers I presume.
Zooming on MacOS in Firefox is awful - or least was a few months back when I last checked. It doesn’t smoothly zoom but just bumps font size - complete deal breaker for me.
Also video playback can be less smooth occasionally.