You have this setting 'View > Sort by > 'Threaded' This means when you click on a collapsed top thread, it will open all the emails in that thread in separate tabs. This is not new. It has been a function in Thunderbird for eons and is the quick method on opening everything in the thread in one go.

I suggest you do either of the following. If you want to use : 'View > Sort by > 'Threaded' Then click on the > chevron shown on left side of Subject to expand the thread and then select the individual email. OR Set this: 'View > Sort by > 'Unthreaded' All emails will be in a list form which you can sort by date. then click on email in list.

Tools > Settings > General > Reading & Display 'Open new messages in' This option detemines how any email is opened when you use a double click. By default it uses 'A new tab'

It has nothing to do with clicking on a top level email in a collapsed thread.

The reason it started a few weeks ago is probably when you upgraded to the new supernove 115* version. It has auto set up new folders to use a 'threaded' option and for some people it reset all there folders to use 'threaded' which has caused much confusion for some people who never use it.

To switch off this default which gets auto applied in new folders:

• Settings > General
• scroll to bottom and select 'Config Editor' button
• In search type: view_flags
• 'mailnews.default_view_flags' 0=unthreaded 1=threaded
• Click on the far right pencil icon to edit
• Remove the number 1 and type 0 (zero)
• click on tick icon to save.