Security News > 2021 > October > Mozilla upgrades older Thunderbird clients to the latest version
Mozilla is rolling out a forced upgrade for Thunderbird 78.x users, getting everyone aboard version 91, the latest stable release that came out in August.
If you were sticking with version 78.x thus far, it's likely that you were doing so for reasons of stability and add-on compatibility.
The former shouldn't be a concern as version 91 is very stable, with Mozilla's engineers having worked out any important bugs for two months already.
Users are advised to check if their add-ons have released a version for Thunderbird 91, as the add-on system has changed dramatically in the new branch.
Version 78.15.0 is scheduled as the last release for the sun-setting branch, but it's possible that Mozilla will extend support for it.
As of October 5, 2021, 85% of Thunderbird's user base were using client version 78, with only 9% having made the manual upgrade to 91.