After updating to Thunderbird 78.4 I found that KeeBird was no longer functional. For now I’ve reverted to Thunderbird 68.12.1, but I’m wondering whether the add-on is being updated to work with current Thunderbird releases. I’m assuming it’s possible, since the Kee add-on is working for Firefox and I gather that the major change from 68 to 78 is adopting current Firefox architecture. (I confess I don’t understand the details.)
Unfortunately, Firefox and Thunderbird have diverged significantly, so it is not quite that simple. You can follow https://github.com/kee-org/keebird/issues/30 for updates.