A browser will usually check to see if another instance of itself is running, and will refuse to start (or use your regular profile) if it believes that this is the case. It does this to ensure consistency of its database (bookmarks, browsing history, etc.) which it can't guarantee if another browser is modifying the same data it is reading from or writing to.
A browser will check that it is not stepping on its own toes by checking the existence of a lock file. A lock file is created whenever a browser starts to run to warn other potential instances of itself not to start running.
However, problems may arise if the original instance of the browser crashes or ends abnormally, and does not get the chance to remove the lock file. The lock file is located at
Firefox: ~/.mozilla/firefox/<profile-directory>/lock Netscape: ~/.netscape/lock
These file are symbolic links (not regular files) that will point to the host and process-id that created the lock file. You can glean this information by doing
ls -l <lockfile> Example output: lrwxrwxrwx 1 smith wesson 18 Jan 12 15:54 lock -> 12.34.56.78:+21321
If you cannot run a browser because of this exclusivity constraint, first make sure that there really isn't another instance running on another terminal or hiding on your desktop. You can also check to see that it really doesn't exist by logging on to the host the lock file claims the browser it is running on and doing
ssh 12.34.56.78 -l smith ps -p 21321
If this process exists, you can either find where it is displaying to and stop it, or you can kill it by doing
kill 21321
If the process does nto exist, or the lock file is still there, you can now remove it:
rm <lockfile>
Try starting your browser now. If you still can't start it up, Email the IT staff.