We implemented this in the Daily build of the desktop app last year, using a staging environment for Firefox Sync. But Firefox Sync is called Firefox Sync because it’s built for Firefox. Thunderbird profiles, in comparison, have a lot more data points. This meant we had to build something completely different. As we started to spin up Thunderbird Pro, we decided it made more sense to have a Thunderbird account that would manage everything, including Sync. Unfortunately, this meant a lot of delays. So Sync is still on our radar, and we hope to have it next year, barring further complications. Source: https://blog.thunderbird.net/2025/09/state-of-the-thunder-mo...

In other words, it was more work to adapt Firefox Sync than they thought at the beginning. It's still actively developed so finger crossed it's coming soon.