I want to share what I do most of the time: all the solutions I've tried and work can be found here. Just patiently troubleshoot it. It is most likely a corrupt pref.js file.
The causes: to me still unknown, but the last two times I've had this I see that the history in package update includes mysql updates. Since this happens at first startup, i assume it is always related to updates.
Good practices: I have comed to save a lot of time by doing this:
1) backup the folder where my email is (thunderbird's profile) using deja dup + ubuntu one. If it is a prefs.js corruption it can be quickly restored, and thunderbird just needs to be restarted.
It can be used to restore email as well.
2) Always keep the profile in some other place that is not the default location. In the same link i provided there is more information about how to do that. This is to avoid thunderbird from "forgetting" your profile and it is easier to backup if you simply locate it with your documents
Hope it helps