Yes, this auto-capitalization is annoying... Is the problem that Firefox is autocompleting the email address from a previous capitalized entry, or is there a problem turning off upper case even if you are typing a completely new entry? If you prefer the keyboard to always default to lower case, you can turn off auto-capitalization in the settings for your Android keyboard. For example, for the "Google keyboard" on the latest Android, it's under: Settings > Language & Input On the right-side of the default keyboard there is a button with 3 horizontal sliders. Tap that and uncheck the box for Auto-capitalization.