Firefox profiles have worked really well for me for years. I only have two complaints. (a) I like to configure my tools, and setting prefs via user.js has just never really worked - anything there is ignored. Probably it's just because of my particular setup, but I eventually gave up on it. (b) It would be super nice if profiles could [edit:] share common prefs. I certainly don't expect the developers to make it a feature, and it seems like fixing (a) and using symlinks would do it. But overall, yeah profiles work great.