Great work! I recently started experiencing issues with Minimize to Tray (revived). This is a great replacement. Suggestion: The Opacity / Transparency toggles on TB window [active (focused) | inactive] rather than hotkey only.
This review is for a previous version of the add-on (0.3.5.3.1-signed).
I was using Minimize to Tray Revived (MinTrayR), but it kept disabling the close button on Thunderbird (usually after waking up from hibernate). This has the same functionality, but adds the ability to use a hotkey to minimize to tray. As long as this doesn't show the same bug, it will be a perfect replacement.
Update after 3 days...
It's still working perfectly through 3 or 4 hibernate and wakeup cycles and another dozen or so sleep cycles. It's perfect.
This review is for a previous version of the add-on (0.3.5.2.1-signed).