Since Nebula was released BirdieSync causes TB to regularly throw up the following error: Thunderbird is already running, but is not responding. The old Thunderbird process must be closed to open a new window.
If I disable BirdieSync from add-on manager and re-enable they then cease until I close & re-open TB again.
Sorry this has been going on for ages and I haven't had time to report as I was getting round to a clean TB install as Nebula threw up loads of other problems when using my original profile. Only did clean install recently with a fresh profile; all the TB niggles are resolved but the BirdieSync one persists. Just updated BirdieSync to latest over the weekend thinking that may rectify but it hasn't.