I auto-sort and store my e-mails in a local Thunderbird # folder tree, and I use tags to organise priorities.
It would be helpful if tags had an option to work across all sub-folders in the tree, and ideally be selectable in combination.
Identifying all the e-mails in my many folders that are both 'Important' and still 'To Do' would be much simpler if I could select the root folder, filter by 2 tags, and see all the output in one place.
But even making single tags work that way would be a big improvement over wading through each sub-folder separately.