In compose, since installing the latest version of Windows WIN 10 rel 1803, Thunderbird has become almost unusable, also when TBird is open other apps are very very slow, close Tbird and they work fine, is this a memory leak, seems like it from my experience. The mouse functions are almost unusable, to edit text and use spell checker, bold, font size etc takes multiple clicks, Tbird freezes then eventually frees, but it a real pain, note have been using Tbird since 2001