Last week I saw there was a Thunderbird update waiting for me to install in Update Manager. It updated from v1.45.5.1 to v1.45.7.0 (both listed as ubuntu 16.04.01)
Since then, whenever I start Thunderbird, it opens twice – once in Inbox, and again in Calendar.

I have always kept these two tabs open (in just one copy of the program), but it’s only since the update that it has started to run Thunderbird twice, making me close down one of the windows.
Has any one else experienced this at all?

Does anything here help?

https://bugzilla.mozilla.org/show_bug.cgi?id=531588

1 members found this post helpful.

hydrurga, thanks yet again. You seem to know how to find the right places to look for these issues.
Although the post in the link was for Windows and about three years ago, it looked like the same symptoms.

It suggested various options about closing the additional tabs/other instances of TB, to end up with only one open, then closing, as TB was apparently remembering too much about previous settings.
I've noticed that closing TB and reopening, it starts again immediately, so I assumed it doesn't unload and as I don't know how to do that in Linux, I kept rebooting.
After more than a dozen different attempts at this, TB is (for now anyway) starting up in only one instance and with the two tabs I want - Inbox and Calendar.
Thanks.

I must have the Google-fu. ;-)

Glad it worked out, paxolin!