There are many many sites on the net trying to get around the lack of a linux compatible plugin for Garmin Connect. Some use scripts to grab the data and upload it, others are running virtual machines, but there is a quick and easy solution that doesn't require coding. From the searching I have done, I haven't seen anyone else do it this way, so I thought I would post it.
I was running a Windows via VirtualBox on an Ubuntu host to get the plugin to work, but found it easier to run the Windows version Firefox via Wine and it works perfectly. Sure its not a native Linux solution, but it works for all models I have tried.
Here is my write up.
http://dropoff.tumblr.com/post/505336492/garmin-connect-running-on-ubuntu-linux