Yesterday, I upgraded my Ubuntu 9.10 to Ubuntu 10.04 beta 1 (Lucid Lynx). Lucid comes with Thunderbird 3. After upgrading, using Thunderbird 3 once and rebooting, Thunderbird started up with empty profile and showed the account creation window. When I looked into the home directory, I noticed that my .thunderbird is moved to .thunderbird.upstream and the new .thunderbird is created with an empty profile. Moving the .thunderbird.upstream back to .thunderbird will work fine the first time and on the next startup, Thunderbird will repeat the same and start with an empty profile.

After banging my head against this issue for a while and some help from #ubuntu-mozillateam, I figured out that Google Contacts addon is causing this problem. When Thunderbird starts up, Google Contacts addon creates a .mozilla-thunderbird directory which is the directory for Thunderbird 2. Thunderbird 3 doesn't like this directory and it does all this renaming stuff. Uninstalling the Google Contacts addon fixed this issue for me.

The lesson that I learned from this experience is to start Thunderbird in safe mode, thunderbird --safe-mode that will help you to isolate issues that are caused by faulty extensions.