I still use Netscape 4.7 mail (MacOS9) just because it's the only mailer which allows to swtich between folders through a combo box. I've never used the previous Mozilla/Mail but I know that it has the same feature. Now it seems Thunderbird doesn't have it anymore.
Thunderbird allows to completely switch off the folder pane but this doesn't make much sense if no alternate way to select folders is provided. The solution with the combo box is a simple and effective solution occupying very few screen area. Therefore I'd really like to see this feature in Thunderbird again. I'd switch to use Thunderbird on MacOS X if it had this feature.