I need to add a new tab to a Page (Main, Behavior, Todo, ...) but I don't want to add it to the end but insert it between "Main" and "Behavior".
Is there a insertFieldToTab instead of addFieldToTab or something else to custom re-sort the tab list?
Hmm off the top of my head and a quick look I can't see anything. In 2.4 I can see a changeFieldOrder() function on FieldSet which may be what your looking for.
Not completely sure if these will work but I think they may $fields->insertAfter(), $fields->insertBefore(). The tabs in the CMS are standard form fields so you might be able to move them round using those.
If you have any luck would like to hear what worked.