I run multiple Firefox profiles concurrently. Using simultaneous profiles allows me to, for example, have both my work and personal web mail accounts open to the same site simultaneously. It also allows me to obscure my browsing habits slightly.
This worked fine up until my upgrade to Firefox 1.5. Suddenly, even if you run firefox as 'firefox -ProfileManager', it won't start the profile manager. It will start up the last profile you used. This is stupid, broken behavior. But the firefox developers think it's ok, because "Profiles aren't for users, even for advanced users. They're for developers." Whatever.
To get firefox to actually do what you tell it to (run the profile manager, so you can pick which profile to start up in the new firefox window), you have to set an environment variable "MOZ_NO_REMOTE=1". Do that, and it works again. It's possible that to someone nerdier than I, this makes perfect sense. But I doubt it.
The fact that remote firefox interaction (read: AIM wants to open a webpage) interferes with profile management should not be the user's problem. One profile is already the default, open links in it and don't mess with this remote crap. If that profile's not open, start it. Voila.