You need to ask Garmin about this.

Also installing Add-on Compatibility Reporter might force enable some features

Thanks danieldesira, installed "Add-on Compatibility Reporter " and now the garmin communicator plugin is running in Firefox 5 beta along with other plugins that wouldn't work prior to installing Add-on Compatibility Reporter .

I could not find the "Add-on Compatibility Reporter." As an alternative, I started Safari (on same PC) just to try anything to register my Garmin. It required me to download a new exe for the plug-in. Ran install, which required closing Safari. Install automatically started default browser (Firefox) which now works perfectly.