You are welcome.
I still don't understand how opening webmail in a browser, which is supposed to do every thing outside your own computer, empties your local Thunderbird.
To get that done the server used by Thunderbird must have been emptied. That can be done by another email client connected to that server which uses the old POP protocol which may copy and delete all mail from the server. Nowadays one usually uses IMAP which synchronizes which the server.
But logging into webmail on the source server will never do that.
Thanks