Comment 37 for bug 1040313

Yes, -profile will still work, I'd forgotten about that. In that case, please use this then (which means you also don't need to touch profiles.ini, and you don't really need to have the profiles in ~/.mozilla/firefox). Although it's unfortunate that we've ended up with a solution that relies on the use of -no-remote, when the chromeless webapp runtime provided with Firefox doesn't depend on this...

As for copying the extensions folder - this would only work reliably if Firefox isn't running, wouldn't it? (else you could do the copy mid-update, or mid-install). But it's odd that in a "chromeless" mode that the addon manager chrome is still exposed via the menus anyway. With the upstream chromeless runtime, there really is no chrome at all (including addon manager). Because there is no addon manager, they've disabled addon support entirely in their webapp runtime (although we re-enable it in our Ubuntu package for system-wide addons in order to make language packs and the menubar work, and because system-wide addons don't depend on the addon manager chrome. But user-profile addons remain disabled).