Long, long, ago in a galaxy far, far away we implemented bug 475053 to suppress multiple concurrent HTTP auth prompts. Having a flurry of window-modal prompts appear when opening a browser/page is terrible UX, so we serialized the authentication. Only 1 prompt will be shown at a time. We need to carefully undo a bit of that for tab-modal prompts... There should still only ever be 1 prompt shown in a tab, but we need to allow different tabs to each be showing a prompt. Otherwise switching to another tab and doing something that requires authentication will just silently hang. [This is actually a bit of an existing bug, since the current prompts are window-modal but not app-modal... Trigger a HTTP auth prompt from one window, then switch to another window and try to trigger another. It will get stuck in a loading state with no prompt shown until you handle the prompt in the first window. Tab-modal prompts are more likely to cause this condition, since they're easier to ignore/miss.] I think fixing this will mostly involve changes in nsLoginManagerPrompter's _doAsyncPrompt(), as invoked from asyncPromptAuth() there. Basically, instead of a single global queue (_asyncPrompts), each tab should have its own. Or some solution isomorphic to that. Doing this results in a few other problems to solve, which I'll file as separate dependencies. TBD if they can actually be fixed separately, but the general architecture should at least account for them.