I uninstalled the formerly disabled XMarks plugin, however only the restart seemed to fix it.

It's apparently related to moving a folder to a different area of the toolbar. Only that folder which i relocate (generally under the location bar icon for easy drag and drop) does not show new or relocated icons within. Every other folder shows them fine. Restart of Firefox corrects the behavior, until moved again.

For now, the temporary solution would be; Relocate any and all new or existing folders, restart Firefox, then go about your business. Until it's moved again, this corrects the issue.

no idea about bug reporting.