You don't say what you consider to be "the setting", but when you go to <Account Settings>, each account has a "Set as Default" button, and you will then see a star by the designated default account. Is this not what you see?

Hi Lin,

Yes, that is what I see, however, when I set my desired email account as default, it always reverts back to the Gmail account when Thunderbird is restarted.

So, is the little star moving to your desired account, but then when you reopen Thunderbird, it's back to the Gmail account? And the "Make Default" button's grayed-out property reverts as well?

Do you notice any other preferences not sticking? I ask because the Default Account designation is stored in the prefs.js file, located in your Profile. If one preference is not sticking, I would expect to see other properties not sticking, either.

First thing to try is to restart Thunderbird in Troubleshooting Mode. Go to menu item <Help | Troubleshoot Mode...>. Let it temporarily, not permanently, disable all your add-ons. After it restarts, try changing the default account again. If it's successful, that could indicate a misbehaving add-on.

Please let us know the results. If that doesn't do it there are some other things to try.