Review for ThunderKeep by Era
Rated 4 out of 5 stars
The button must be added manually: right click on the toolbar, Customise ..., Drag and drop the Thunderkeep button in the toolbar.
I updated the project and fixed the issue in this new addon
https://addons.thunderbird.net/thunderbird/addon/google-keep-tab
To create your own collections, you must have a Mozilla Add-ons account.