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This add-on makes Thunderbird incredibly customisable. The only limitation is the user's own understanding of XUL, which in my case is not comprehensive--but in principle you can do anything. Absolutely brilliant--and it does work with TB 45. I had been stuck on TB 2.0.0.23 for seven years because of all the add-ons that TB 3 broke, but Custom Buttons just lets you get the functionality back.