So here more than 6 years later, did they act more swiftly or pursue new ideas? The development pace seems unbearably slow.

Judging from the side it seems like there aren't a lot of developers and the current few have their favorite subsystems, those get almost all of the attention. The rest is kept as-is and does not progress. I also don't know if there's a trivial way to find how many external contributions they get from BugZilla, if they even get any?

Total mail UI/UX overhaul, total backend overhaul allowing implementation of new protocols (first new protocol was Exchange), an Android app and monthly release cadence vs yearly. And that's just to name the ones that immediately come to mind. Have plenty of users yelling at us for changing too much.

Take a look at https://blog.thunderbird.net for a play-by-play. But we're doing a helluva lot.