I used Send to GPS several times recently with Firefox.
Perhaps Garmin's plugin does not work properly with your version of MacOS? Maybe contact Garmin.
Good luck.
That's not a bad idea, contacting Garmin, but the interesting thing is that I can send to my GPSr all day when I log into Garmin's myGarmin site. Additionally, I believe I have also successfully sent waypoints to my Garmin using Google Maps. ?? I'm wondering if there is a protocol (or API, or something) that Geocaching.com needs to update on their back-end to accommodate for the new Germin GPS units? I have contacted GC.com tech support already (still waiting a response) and will be contact Garmin too.