I wanted to say that I like this idea, and perhaps the option to group them by which bar they normally go on (e.g., main navigation bar, add-on, bookmarks, etc.). Looking briefly at the current button palette, I don't see any way to change the sequence now.

You could submit a feature suggestion here: http://input.mozilla.com/en-US/feedback#idea

I have long detested this glaringly obvious omission and have complained before. SeaMonkey has this facility, very nicely done with sort options, so I cannot see why Firefox and Thunderbird don't too - please hurry up and extract the digit, Mozilla! Or perhaps some clever developer out there could address the situation with an add-on? PLEASE!

UPDATE -- Look what I just found! https://addons.mozilla.org/en-US/thunderbird/addon/sortcustomizationdialog/

It's working in FF and Tbird, and must be relatively new because I've hunted for this loads of times before, and I've only just this minute found it. It's the same as SeaMonkey, so enjoy, all you tidy-minded people out there.