Please install History in Threads beforehand. Otherwise it won't work
Deleting a thread will remove all the visits within this thread from Firefox's browsing history. NOTE: this deleting operation is NOT recoverable.
You can also selecting multiple threads and delete them together.
After installing, close and re-open sidebar of 'History in Threads', a new menu item "Delete" will show up in the context menu.
After uninstalling, restart browser and the menu item "Delete" will be gone.