Flat Folder Tree

5 ਤਾਰਿਆਂ ਵਿੱਚ 5 ਮਿਲੇ

ਵੱਲੋਂ Gyrocode.com ਅਗ 11, 2017 ਤੇ · ਪਰਮਾਲਿੰਕ

I am surprised this hasn't been mentioned before. There is much simpler solution available that no longer requires add-on.

See this post for more details and screenshots:
https://www.gyrocode.com/articles/how-to-hide-gmail-folder-in-mozilla-thunderbird-using-imap/