Firefox sync really is neat, but be aware that for the last several years it has had a security flaw, one which the Firefox team has no interest in fixing (in fact, they consider it a feature!): your passwords and other private data are encrypted with your Firefox Account password. That would be fine so long as a) your Firefox Account password is truly secure and b) Mozilla can never see it. Unfortunately, they can: they serve an HTML page to log in to your Firefox account, which loads JavaScript from their servers. At any point they can start serving JavaScript which sends all passwords to their servers.
What is so very, truly disappointing is that they used to have a great, easy-to-use Sync security system which was immune to this sort of attack, but they removed it.
Anyway, I cannot advise using Firefox Sync for private information anymore.
I trust Mozilla to write Firefox and I trust Debian to package it. I do not trust Mozilla to be trustworthy on a live and ongoing basis.
Can I associate one login/password pair with multiple domains?