I use the Startup Master add-on. It inserts a "splash screen" password dialogue box into the Thunderbird start-up sequence, which you have to fill in just the once. It has a minor cosmetic privacy value in that you don't see any content at all until this is correctly filled in.

https://addons.mozilla.org/en-GB/thunderbird/addon/startupmaster/

Given that I have about 11 accounts and two networked calendars, you can imagine that one prompt for each would become tiresome very quickly.

thanks, but third party add-on for a security thingy sounds not so secure. I assume this is Thunderbird bug and I hope it will be fixed. For now I'll stick with unchecked "Check for new messages at startup"

I was suffering from the same problem.

I currently have 10 email accounts set up in Thunderbird and received 6 master password prompts when starting the program (Thunderbird version 45.3.0 running on Windows 7). I was interested to see that it was 6 password requests and not 10, but looking at the troubleshooting information, it appears that the 6 password requests may line up with the 6 Microsoft IMAP accounts that use STARTTLS connection security (the 4 others use SSL/TLS connection security).

Thank you Zenos for your link to the Addon which I have installed and it has fixed the problem. As geotavros has rightly stated, it should not be necessary to turn off the "Check for new messages at startup" or use an Addon to get round a Thunderbird bug. I will have a look at Bugzilla when I have more time and see if there is a bug registered fro the issue.

Incidentally, I use Waterfox for my web browser, under both Windows 7 on my laptop and Windows 10 on my desktop and have not had any issues with the similar Master Password facility there.

Thanks guys!

Problem persists in 45.5.1.