This addon has inspired select functionality to be added to core Thunderbird in v70+. Pick column Select Messages from the columnpicker.
Also included is 'no unwanted selection', meaning unselect in the column will prevent selecting anything in that folder on junk or delete or other message move actions.
Thanks for making this! Some suggestions:
1. You should register "selectCol" as non sortable, on init, because it 1)doesn't actually sort 2)confuses the Group By Sort view;
2. It would complete the functionality if you could add an onclick listener to the column header and then toggle between Select All and Clear Selection (maybe with a tristate icon - none, some, all selected).
3. A tooltip on the column header would be good.
4. The optional large font is good, but please add an option for the regular sizing, ie a css rule that sets the icon at 16x16, the max for all other stock icons.
Thanks for the excellent suggestions. I've added them in 1.2.