What is the "separate" folder? Whilst Thunderbird can add stars itself, it's possible your mail provider is doing this for you. Google have an arrogant way of assuming they can tell what you think is "Important" and mark it thus and pop it into an "Important" folder. Hotmail/outlook has a similar feature called "Clutter", IIRC. Bah humbug. How have you tried adding a signature? I find that one signature never quite does the job on its own, so I use a signature switch add-on to help choose between multiple signatures and therefore create all my signatures as html files, as required by that add-on. https://support.mozilla.org/en-US/kb/signatures https://addons.mozilla.org/en-GB/thunderbird/addon/signature-switch/