I use Firefox's quick search by typing a search engine's keyword to address bar preceded by the search query.
But there are some sites which permit only the logged-in users to use their search engines.
I have multiple accounts for that site and all of them are logged in using MultiFox.
The problem is that whenever I try to use a quick search by search engine's keyword - a tab gets opened with that site, but I am unlogged there. Thus, the search query fails: I have to log-in again (although each of my accounts is already logged in) and re-send the query. I use that feature so often, that it pisses me off.
If you'd add a feature to set an account as "default" - that would solve lots of issues, not only this one, but the issue with incompatibility with "Gmail Notifier" extension that is unable to check gmail's inbox even for a single account if MultiFox is active.