Thought this might be helpful to someone: I found out that mozilla 1.1 cannot process certain entries in the prefs.js file any more. My prefs.js contained a couple of entries of the form "mailbox://nobody@Local Folders/Sent", e.g for "mail.identity.id2.fcc_folder" and other folders. This led weird behavior of the Mail & Newsgroups Account Settings -> Copies & Folders dialog. It also prevented sent messages from being saved (but no error message!). I've never before edited prefs.js, so this must have been written by an earlier mozilla version. By changing these entries to "..Local%20Folders.." I got rid of these problems.
This is related to bug #166130. Lutz
