I used to use floccus and the thing I really liked is you can selectively share bookmark groups. So if you have certain links you want everywhere you can do that, but some sets you might only want in in specific browsers. I do not know if the others that have this.
Stopped using it because of unresolvable problems and not much Dev attention but looks like its picked up again so I plan to get back to it.
someone else can probably give a more comprehensive/correct answer but here is how I understand it. i believe chromium is open source and chrome is mostly chromium but also some proprietary (and therefor unknown) bits are included. whereas firefox is entirely open source, meaning you could compile it yourself and still end up with the same package.