I'm certain others have requested this, but a brief search failed to come up with results, and some came up with ideas for work-arounds but they had parameteres to filter things:
Simply: Make add Favorite folder, is really adding to a list of Favorite Folders. Or make a Recent folders that were last opened, like many other applications use.
I work using MP3tag daily, I am typically using 4 or 5 folders repeatedly. Having a favorites drop down or Recent drop down both in the File menu and on the task bar, would save me a LOT of time.
I have been using MP3tag for 20 years, I think its great.