Due to regressions we stopped delaying the autofill if a master password is set (bug 1538460) so we didn't address the issue of MP prompts from background tabs in this bug.
Bug 1149500 Comment 15 Edit History
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Due to regressions we stopped delaying the autofill if a master password is set (bug 1538460) so we didn't address the issue of MP prompts from background tabs in this bug. I filed bug 1567574 for that.