So I use Thunderbird everyday as it is my only mail reader and I am up to version 12.0 and it is working properly for me. I obtained my copy through YaST and not a build service, but if there is a bug report required, I would suggest that you should file one with all the info you placed here.
Like James, I run Thunderbird constantly.
I recently updated to 12.0 on my 12.1/KDE 4.8.2 desktop, mozilla repository.
It seems to be checking mailboxes on startup, I see the activity in lower left corner of window.
I have the ViewAbout plugin installed.
When I View-View/About-about.config and filter on “start”, I see
server is my “Local” folder, server2 and server3 are active mail accounts
It appears you are running a desktop.
I have experienced your issue on startup with a laptop, because Tbird was starting before network(wireless was up)
And then it is selected and loaded from YaST. This may be exactly the same thing you have done, though I did not understand that if true. But as said before, I have had no problem with Thunderbird 12.0 and further, if I had installed it from a Tarball and found it to work, I am not sure I would do anything else until version 13.0 (or maybe they will jump to 14 if superstitious). I am not sure which way you used first, but perhaps installing from different sources has messed up something?
Thunderbird 13 is the next version, currently in Beta.
Anythings is possible, but I run all 3 versions of Thunderbird I use with separate profiles, and the release version was a new profile that has only the Lightning extension installed.
So there are no security updates in Thunderbird 12.0 to bring it into the Update repo?
So I guess I would look at the following setting in Thunderbird called: mail.startup.enabledMailCheckOnce and see if set to True?
Thunderbird / Edit / Preferences / Advanced / General Tab / Config Editor / search on **mail.startup.enabledMailCheckOnce **and make sure it is set to true. I think if you double click on it, it toggles its value. After making a change, exit Thunderbird and start it up again. If already set to true, then it may be bug report time.