When you have a folder open with a selected message and quick filter toggled on then after escaping filter with CTRL+F to focus it and ESC to close the bar further input is still being processed. In my case I wanted to untag a message when I noticed filter was on (as in the bar was visible with no conditions applied) and switched it off. Then I entered tag number and expected the message to be untagged. Instead quick filter processed that number and all but one email in the view was suddenly gone because no other mail matched this condition. I would expect that focus in email list should be restored after escaping the bar instead.
Possibly related to bug 571280 (similar focus issue).
Merike do you also see this wihout lightning ?
Yes, I also see this with no addons enabled.
It looks like this is fixed in Nightly for Thunderbird 8, by the patch for bug 564328. (The shortcut for Quick Filter has also changed to Ctrl+Shift+K.)