It's time to phase out the gray tab replace it with Tabzilla. https://github.com/mozilla/tabzilla Thanks! Please ping with questions!
Andrei, currently only for en-US.
:sancus: Do you have an update on when this tab can be added to the Thunderbird site?
This is actually fixed on most pages except for the front page, but I'm not going to fix it in PHP since it will just happen for free when these pages are ported to bedrock, which is Soon(tm).