Are you sure it is Thunderbird and not your anti virus that is munging the password?

Yes, I think it is Thunderbird. I changed the server settings as instructed and found no field to enter the new password. Then I clicked "get mail", expecting to be asked for the new password. This happened, a window opened in TB "Enter your password for user@domain.com on imap.provider.com:" Below this sentence an empty field, ca. 50 digits long. But whenever I typed my new password, the field stopped accepting my entries after 8 digits - I see 8 big black dots and cannot enter the last two digits.

Tools menu (alt+T) > options > security > saved passwords and click the show passwords box to check what is being saved...

I do not know that answer, perhaps it is a limitation..... but that will confirm it for both of us.

I'm routinely using one password of 10 characters length, and another at 13. Both in Thunderbird, and on both Windows and Linux. I can't reproduce your problem.

I recall some years ago seeing a system that replaced the string of asterisks that represented your password by a new shorter string of asterisks; I suspect it was a deliberate ploy to make it harder to count the number of digits actually in use. But I don't think Thunderbird does this.

Thunderbird asks for the password a) when you're setting up an account or b) when the password has failed and is assumed to have changed. You can't just add it to the account settings; you have to wait for Thunderbird to request it.

@Matt: It took me some time to find the "show passwords" box, because in my version, it is hidden in "edit - preferences - security - passwords - saved passwords". But I finally got there and saw that still the old password from the previous provider was saved there. And that indeed had exactly 8 digits. So I clicked "remove" and deleted this old password, thinking that this might be the reason for TB to accept only 8 digits. But, @Zenos: Then I tried again to "get mail", was asked for the new password, and had the same problem as before. Entry stops after 8 characters.

@Both: Thanks nevertheless!

Edit: Then I closed TB completely and opened it again, so that it tried to get my mails automatically. And again I was asked for the new password, and this time it accepted all 10 characters! Problem solved!

Thanks again to both of you!

You might want to bookmark this reference chart since you do not use Windows. Most advise and help pages refer to the menus as shown in Windows. The menu layouts vary per operating systems.

http://kb.mozillazine.org/Menu_differences_in_Windows,_Linux,_and_Mac