I don't know if your patch fixes it yet, but I have the problem with NNTP as well.
But there the newly entered credentials are not accepted for saving.
I get the exception:
10:11:11.443 NS_ERROR_FAILURE: This login already exists. 2 LoginHelper.jsm:1752
If I then select another article in the same group, I get a login dialog without a "save option" and can then work temporarily.
But if I select another group I always get the first dialog with "Save" option. With the same result. Even after a restart of the TB.
What helps is to delete the userName entry manually from the Prefs.js.
If the problem persists there, I can also create a new bug for it.