Rated 5 out of 5 stars

<add 20211109>
Thanks for the reply, I tried based on your reply and finally got it fixed.

Are the add-ons lost? → NO
Are the add-ons enabled? → YES
Reinstalled add-ons → disabled after reboot ( not changed)

Finally, I once disabled the add-on, and then enabled it again, so that it was enabled even after restarting thunderbird.

<original>
In thunderbird91, this can change the icon at the time I add the add-on, but when I restart thunderbird, it will go back to the default black and white icon.
Very disappointing.

This review is for a previous version of the add-on (1.0.4). 

Have you checked whether the add-on is still enabled/installed after restarting thunderbird? There's a bug with thunderbird (not the add-on mind you!) that it removes add-ons after a restart of the program.
Re-adding the add-on usually fixes this.

Edit: good to see you got it fixed!