I've been using #Betterbird for a few months now while I'm trying to migrate away from Outlook. It takes some time for testing and finding new work flows, but so far I like the improvements to Thunderbird.

Biggest challenge is finding a good way to mark emails for follow up, so they are added to my task list. BB is creating a copy, losing the link to the original email, but I might have found an add-on now. I'm open to suggestions!