There is a Suspend background tabs
extension. (I ran into, seemingly, a compatibility issue with it, on SeaMonkey, so stopped using it.)
Not releasing (hanging) from memory is different from "taking a while" to exit from memory. The way Mozilla closes now is to first remove the UI, then it continues its shutdown in the background. During that time, your Profile remains locked. Once finished shutting down, the lock is remove & you can reopen the Profile. Depending on how much memory your have racked up, what plugins, ... may have been open, can affect how long it takes it takes to release from memory. As it stands right now, I'm using 1.5 GB of RAM, & unless I outright kill (seamonkey.exe), it is going to take a bit of time before everything unloads (though thinking in any case it will be 30 sec).