Comment 38 for bug 308605

I fear we now have at least two bugs in this one report.
1 Firefox not releasing a lock, and thus blocking a new instance. In this case, no amount of waiting or starting new instances will solve the problem, just drastic methods like restarting the computer will.
2 Firefox releasing a lock after a new instance already found it locked and therefore considers itself blocked. In this case, you merely have to stop the new instances and start it again. The problem here is that you can't tell it to retest the lock.

Problem 1 would appears to be an implementation error; problem two a design error.