If you were wrong about Brave, your concerns about Firefox and Mozilla could be wrong as well.
There's nothing wrong with the all-purpose heavily featured approach of stuff like Chrome and Firefox, and I get that other people like Sync, I just really wish there was a totally stripped down basic internet browser I could trust.
[0] https://www.mozilla.org/en-US/foundation/annualreport/2016/
Do they? I just skimmed the source and could not find anything that hints at that (and implementing it the way you said it is makes for much more complex code..)
For the average user—think your uncle who's about to ask you tech support questions this weekend—using Firefox Sync is way better for their privacy and security than reusing the same password everywhere. And for those of us who have and use password managers, Firefox Sync is not on by default—unlike Brave's attention-tracking or Chrome's Google logins.