Rated 4 out of 5 stars
Although stated "compatible Thunderbird 13-45.*", version 0.9.1 runs in Thunderbird 52.0.1 32bit.
The addon just adds a button "Google Keep" to the menu bar of Thunderbird. If you push it, a new tab in the Thunderbird main environment is opened, that shows a browser window to Google Keep. You have to login in this browser window at the first call, then your credentials are saved. No settings in the addon itself, really simple.
And that's why I use this addon:
Websurfing with Firefox, I open links as tabs, so my tab bar is really filled.
Not so in Thunderbird, where I read my mails mostly in the preview window - so I have no or just few tabs in Thunderbird.
As I use lightning (connected to my Google Calendar), my Thunderbird becomes a "single point information center".
To enhance Thunderbird further to a single app mail/calendar/notice point, I installed this addon.
What I miss: some kind of addon to connect notices between XNote/Xnote++ and Google Keep