Yettwasezmel 5 ɣef 5 n yetran

sɛur Gyrocode.com di Aug. 11, 2017 ·

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/