http://www.cs.wisc.edu/csl/doc/faq/m...removelock.faq
1.11.5. The Profile Manager Tells Me My Profile Is Already In Use But I Know It Isn't. What Should I Do?
If mozilla tells you your profile is already in use but you know it isn't, it's probably because mozilla crashed and did not remove it's lock file. You'll need to remove the lock file by hand.
For the windows version of mozilla, the lock file is U:\.mozilla.w2k\User\puegzo40.slt\lock
The location of the lock file is slightly different for each user under UNIX. It is located at
~/.mozilla/login/XXXXXXXX.slt/lock
where login must be substituted with your login name and XXXXXXXX.slt is some random directory name that you will need to determine (by looking in ~/.mozilla/login).
Remove this lock file and you should be good to go.
Here's an example for a user named "joeuser":
cd ~/.mozilla/joeuser
ls
mzti5x2o.slt
cd mzti5x2o.slt
rm lock