In a blog post, Mozilla explains its e-mail client will get a new user interface this year. Additionally, a lot of effort will be dedicated to making Thunderbird faster, by addressing UI-slowness and general performance issues:
Our hires are already addressing technical debt and doing a fair bit of plumbing when it comes to Thunderbird’s codebase. Our new hires will also be addressing UI-slowness and general performance issues across the application.Users can also expect some new features like better Gmail support, as well as an improved notifications system. After being on life-support for many years, it's nice to see the phoenix rise from the ashes.
This is an area where I think we will see some of the best improvements in Thunderbird for 2019, as we look into methods for testing and measuring slowness – and then put our engineers on architecting solutions to these pain points. Beyond that, we will be looking into leveraging new, faster technologies in rewriting parts of Thunderbird as well as working toward a multi-process Thunderbird.