Rated 5 out of 5 stars

I ran into the same problem with 2FA that others have mentioned, but all you need is an add-on that will allow you to open a link in another Thunderbird tab.

There are a couple of options for that. I chose this one:
https://addons.thunderbird.net/en-US/thunderbird/addon/open-tab/

When you get to the screen that tells you to verify your account, just right-click the button and choose "Open in New Tab" from the context menu.

Facebook might give you some hassle depending on which way you choose to verify (a code from an authenticator app is probably the best option), but once you verify on whatever other app or device you're using to do so, close the Messenger tab in Thunderbird, then reopen it. It should load at that point and ask you for your encryption PIN.

Make sure you check the box to stay signed in so you don't have to go through all of that every time!

As for this add-on itself: it does exactly what it says it will! I've got a Messenger icon in my Spaces bar, and it's now open in a tab for me. So far so good!

Rated 1 out of 5 stars

None of these addons that open Messenger in a tab work ever since FB changed their app from Metro to web based. Impossible to sign in for whatever reason. FB only works in proper browsers now. Thanks a bunch Zuckerberg.

Rated 1 out of 5 stars

Did not work with 2FA for me, it opened Firefox instead.

As stated in the description, this simple add-on only redirects you to Messenger; it cannot force inner page links to open as new Thunderbird browser tabs.

If you're familiar with JS, you're welcome to contribute to on GitHub anytime.

Rated 5 out of 5 stars

Addon works great!