Since updating to Thunderbird 17, I've noticed on all of my computers that run it for their email, the program opens and runs well but is slow to close, taking less than a second to open but 5+ seconds to close. That is the same on every machine.
For a comparison, Photoshop CS6 opens in a couple seconds and closes in less than a second. I never had the issue before TB 17 and all machines have fast CPU, tons of memory, and SSD storage.
I hadn't noticed this at all. Do you have large imap folders? It may be taking longer to try and sync folders before shutting down. If you have t-bird set to purge deleted emails on close, this may cause it too. Also, if you use the built in "sync", it may be pushing your booksmarks and stuff to the cloud on close.
I never noticed it up through version 15 and haven't changed anything else other than the update. Interesting that it affects all my machines equally with that email account set. I have indexing off and all of my mail through CableOne is on the Google mail servers so the small delay at startup to download and sync the mail happens then.
It isn't a big deal, just seems odd that it started with the 17 update.