There will be an update to fix this issue soon.

Edit 2019-01-03: The latest Contextlets update should resolve the issue with the checkboxes, which was due to a change Mozilla made in their CSS.

Unfortunately, I'm not sure the icon issue is something I can fix. It appears that Mozilla made a change so that custom icons are no longer allowed in the top level of the context menu unless the icon is hardcoded in the extension itself. The custom icons only seem to work within submenus. I will investigate this some more when I get a chance.