The @thunderbird betas have a rewritten main window UI. This has a lot of accessibility problems. Unfortunately, I currently don’t have the energy to file the bugs myself. Here’s what I found:
1. Too many tab stops. Toolbars should definitely be a one-tabstop container and employ the ARIA toolbar design pattern.
2. Speaking of which, the containers should all be made into proper toolbar containers in the process.
3. The main messages list should be a tree with tree items, since they are collapsible. List and list items are not. Also, there are a lot of focus issues still remaining when deleting messages/threads or when the view refreshes from the IMAP server.
Perhaps someone on the team can convert these into proper bugs in Bugzilla, or maybe most of these are already filed. Like I said, I unfortunately currently lack the energy to do so myself.