When You Can Not Empty Trash In Thunderbird With IMAP Folders
Find Your Profile
Start this whole show by shutting down Thunderbird, then locating your profile. In any Linux flavor I've used, my Thunderbird profile has been located in /home/craig/.thunderbird. In Windows, I'm not sure what to tell you. I know in XP, it was in C:\Documents and Settings\Username\Application Data\Thunderbird, but newer versions of Windows put things in different places. This Mozillazine article about Thunderbird Profiles explains it well.
Find Your Account
Inside the appropriate profile directory (named something like erfbhafu.default) there should be an ImapMail directory, and inside THAT you should see a directory and corresponding msf file for every imap account you check with Thunderbird. Insode of THOSE directories, you'll see another directory called INBOX.sbd, and this is where Thunderbird caches your mail. Here's a listing of what's in my profile directory.
You may find the remnants of past accounts even though you removed them from Thunderbird. These are the ones that I've got left, but there were a few others I found when I was getting ready to write this post. The trick now is finding out which directory belongs to which account. If you've got a folder in your inbox, it will also show up here in the file tree. If I run a
I get a list of all the inbox folders I'd see in Thunderbird. This is how I know which folder is which account. If you've got no folders in your inbox, make one called TEST or something, then it will show up here and you've found the account.
Delete Trash Directory
Also in the output from the ls command, I see Trash.sbd. This is what needs to to be deleted. There's a Trash.msf file as well, but I left it alone and still got the desired result.
Start Thunderbird back up and your trash should now be empty.
The official bug report that I found and used was here: https://bugzilla.mozilla.org/show_bug.cgi?id=632564. Let me know how you made out.