At View → Sort by → Grouped By Sort you can activate the grouping of your mails by the chosen sort method. E.g. when you select the sort method "Date", you get groupings like "Today", "Yesterday", "Last Week", ….
These groups can be (un)folded by clicking at the little triangular icon (in some themes it seems to be a plus sign).
I observed several Thunderbird users that tried to unfold a group by double-clicking at the group heading (e.g. "Today") instead of single-clicking the little icon. I think this is natural behavior known from many other applications. However, Thunderbird won't unfold the group. Quite the contrary: Thunderbird opens all mails in this group. This creates two problems:
- all these mails are now marked as read
- it may take some time to close all the tabs again
After "Two Weeks" the next group heading is "Old Mail". This group might hold several thousand mails. You can imagine that it's no fun to see all these mails opened accidently.
Is there a (config) setting that disables this behaviour? If not, is there an add-on?
When the user double-clicks the group heading, the group should (un)fold (and never open all mails).
I filed an issue: https://bugzilla.mozilla.org/show_bug.cgi?id=803997