Google has a huge collection of data sources (search query correction, document translation engines, the Books project) to refine a spellchecker against.
The open-source solutions have never had access to that volume of data, to my knowledge.
1. On the list of words Firefox gets wrong that I've reported but haven't yet been fixed, LibreOffice gets them all right.
2. On the list of words that I indirectly reported quite a bit earlier that Firefox now gets right, LibreOffice gets one wrong ("heliocentrism", which Chrome also gets wrong, and MacOS gets right).
3. On the list of words that Firefox gets wrong that I have not yet reported, LibreOffice gets them all right.
I don't call Firefox's terrible spelling inexplicable because Google and Apple and Microsoft do much better. I call it inexplicable because LibreOffice does it much better (about as good as Google and Apple and Microsoft do it).
Firefox uses the same open source spell checker, Hunspell, that Chrome, LibreOffice, MacOS, and many others use, but for some inexplicable reason have a terrible dictionary. I've ranted on this before [1] so won't do so again here.
I get the impression that the poor handling of multiple profiles is because they expect you to use multi-account containers instead.
Those are fine if you just want, say, to access multiple instances of the same site without using the same cookies for each, or want to make it harder for cross site cookie tracking, but for most of the times where I want to access multiple instances of the same site I also want separate bookmarks and add-ons and history for each instance.
[1] https://news.ycombinator.com/item?id=24819714