Thunderbird is not fully opening up on my macOS

I think that your Thunderbird startup window retrieving numerous tabs from the previous session is victim of this bug https://bugzilla.mozilla.org/show_bug.cgi?id=2071951 "''Thunderbird hangs on startup when restoring session.json"'' The only solution I know consists in deleting the '''session.json''' file in your Thunderbird profile folder. To open Thunderbird's profile folder on a Mac one must follow this path ~/Library/Thunderbird/Profiles/ in practice: verify that your Thunderbird app is closed, open a Finder window, go to menu '''Go''' and select "Library" in the sub-menu. Now in the Finder window you'll see the contents of the profile folder (named somewhat like xxxxxxxx.default or xxxxxxxx.default-release, ...) Search for the session.json and session.json.backup files, delete both of them and close the profile folder window. Restart Thunderbird which will create a new session.json file. Of course no more tabs will be displayed except for the mail tab with your accounts and message folders. Until this bug gets fixed I advise to close most message tabs each time you quit Thunderbird, in particular those displaying contents of PDF attachments.