To add to that, Firefox sync work well with iceweasel at least in my personal experience. My understanding is that iceweasel is Firefox, just an older version with security patches as needed.
Iceweasel in Debian unstable stays up to date with the latest Firefox Extended Support Release, and Iceweasel in Debian experimental stays up to date with the latest Firefox release. Packages also exist in a separate repository for the latest beta and aurora releases, as well as of the latest release packaged for stable.