If your Thunderbird suddendly stops sending emails mumbling something about you don't have enough permissions to access some files or you shall check your network settings, do not listen to it! Just go to C:\Documents and Settings\User\Local Settings\Temp and wipe all its ns*.tmp files!

Here is unambiguous a bug (including last TB 5.0) – if the last file named nsmail-9999.tmp is created, well then TB is doomed.

This little bug ruined an half day of my life.

Links: Bugzilla