By the look of this bug https://bugzilla.mozilla.org/show_bug.cgi?id=382415

I don't think there is an sort of "recent" or "frequent" selections.

I'm sure that my Thunderbird did offer the frequently used email addresses first until I turned on the Google CARDDAV sync. It does not work since. @Matt I'm not sure if the bug mentioned above is related to my problem.

The drop down list is based on best match to your typing entry. The more letters you type the smaller the list becomes as it filters on your typing of email address. The drop down selections of potential matches are sourced from the Thunderbird address books.

I have always found that when you start to type an email address in a TO field, Thunderbird offers a potential list that best matches what you are typing based on current contacts in address books.

So if you sent an email to a person that was not in an address book AND you do not auto save outgoing address in an address book, then if you wanted to create another email to same person they would not be offered in the drop down list.

Example: If you started to type: gmail then observe the list and you should find it lists all the @gmail.com email addresses.

However, the results should be displayed in a popularity order, not necessarilly an alphabetical order.

So the order of what you see in list maybe most frequently and recently used at the top, but the list itself is not strictly a most recent list, it depends upon what you are typing.