Comment 25 for bug 1012858

David, thanks for reporting your results. Can you retest please and tell us if this reproduces with a curent beta?

If it reproduces, please run the performance profiler:
1. You must be using Thunderbird 68 or newer - betas from https://www.thunderbird.net/en-US/channel/ or current nightly build from https://archive.mozilla.org/pub/thunderbird/nightly/latest-comm-central/
2. Install profiler add-on into thunderbird 68 (or newer https://www.thunderbird.net/en-US/channel/ ) - get the add-on file from https://github.com/firefox-devtools/Gecko-Profiler-Addon/blob/master/gecko_profiler.xpi?raw=true and in Tools > add-ons click the gear to install add-on from file
3. Follow instructions at https://profiler.firefox.com/ (videos BASED ON FIREFOX at https://profiler.firefox.com/docs/#/./videos-intro )
4. Create a profiler URL and post it here.