Before bug 563274 the prompting code was written in C++ and specifically did not attempt to use the checkValue parameter if no checkMsg string was provided. Unfortuantely we no longer have that luxury. In fact, we have to pass an initialised PRBool, so there are six call sites that need to be fixed. I originally noticed this trying to delete a local folder; I haven't actually attempted to verify the fix for any of the other call sites.
Created attachment 453833 [details] [diff] [review] Proposed patch
Created attachment 453836 [details] [diff] [review] Include nsIPromptService::PromptUsernameAndPassword Most callers use nsIAuthPrompt which does not have this issue. (This also goes for the some of the other Prompt function calls.)
Pushed changeset f27e030e9a14 to comm-central.
Pushed as http://hg.mozilla.org/comm-central/rev/51b6b3c86072 to the 2.1a2 relbranch, so this is fixed there (in build2) as well.