I recently resolved what may be a similar issue though your problem description does not list enough detail to be sure. Findings were posted in Regarding Firefox revisions >= 39 and related 6.0 U3 Client Integration Plug-in issues . In summary, I ended up resolving my issues by using Firefox 52 ESR and by making sure that Firefox Options | Applications | vmware-csd is set to "Always ask". My problems were also related to exporting OVF templates, and this solution path made it possible for me to export an OVF template.
