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; gFolderDisplay.COLUMNS_MAP_NOSORT.add("selectCol"); 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.
This review is for a previous version of the add-on (1.0).