Development by Mozilla has been stopped since 2012, which means that Thunderbird has been relying on community support for maintenance and new features, Mozilla still contributing security updates. For details see here: <a href="https:&#x2F;&#x2F;blog.lizardwrangler.com&#x2F;2012&#x2F;07&#x2F;06&#x2F;thunderbird-stability-and-community-innovation&#x2F;" rel="nofollow">https:&#x2F;&#x2F;blog.lizardwrangler.com&#x2F;2012&#x2F;07&#x2F;06&#x2F;thunderbird-stabi...</a><p>Development has been slow since then, but the project is far from dead. I&#x27;m on version 42 beta (latest Firefox is 43 I think) and for example they&#x27;ve just includes the Lighting add-on in the base install. Plus all add-ons that interest me are compatible, like Markdown-here.