I know for sure that there is one (the signed version is new) as I downloaded it and it works fine (with the newest version of Firefox)
I found it here
https://addons.mozilla.org/en-GB/firefox/
by searching for Garmin communicator
This is weird. I just now downloaded the version I found as per your suggestion, and it did NOT work. Same error message. I have Firefox 43.0.1 and a Garmin Oregon 450.
I received the following procedure from the support group at Garmin. It worked and I can again download the gpx files directly. As per the instruction, I went through the process 4 or 5 times before it worked.
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In the Firefox blocked pop up screen, please click the "X", in the top right corner of the "Blocked" message and refresh the browser page while you are on the Plug In Installation page. You may have to do this a couple of times before it acknowledges the plug in installation.
To refresh the browser, please click on your F5 key on your keyboard.
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