What does the lock icon mean on an account in Thunderbird?

"It means that the account uses a secure connection. GMail uses SSL for incoming which is a secure connection. This does not mean that the account is 'locked.'" - Daifne
http://forums.mozillazine.org/viewtopic.php?t=630102&sid=239bcedd71b4c0ae10d1898b8dfa92b0

Creating accounts in Thunderbird for popular email providers - MozillaZine Knowledge Base
http://kb.mozillazine.org/Creating_accounts_in_Thunderbird_for_popular_email_providers

Remove the profile lock file
"The application may have shut down abnormally, leaving the lock in place. To fix this, open the profile folder and delete the file,

• "parent.lock" (Windows),
• "lock" and ".parentlock" (Linux), or
• ".parentlock" (Mac OS X)

For Mac OS X: You need to use a free utility like Onyx or Maintain to turn on visibility for invisible files to show ".parentlock". This file contains no information, and deleting it will unlock the profile.

For Windows: If you attempt to delete the "parent.lock" file and receive the error, "Cannot delete parent: The file or directory is corrupted and unreadable", restart the computer and run the error-checking tool Chkdsk (Windows 2000, XP) or ScanDisk (Windows 98, ME) [1]."
http://kb.mozillazine.org/Profile_in_use