Mozilla/Netscape/Firefox says that my profile is already in use, but it isn't; how do I correct this?
The Mozilla-based browsers (mozilla, firefox and netscape) use lock files in ~/.mozilla to make sure that only one session is using your profile at a time. If your browser crashes or otherwise stops without closing down properly, the lock file isn't removed and subsequent sessions will refuse to use your default profile. You can remove the lock for these applications with the following commands.
clear-firefox-locksclear-thunderbird-locks