This post has been superseded by this post due to Firefox has closed this fix with their latest update.
What I have learned in all this is it is NOT totally true that FireFox no longer supports the Communicator Plug-in, the issue is Firefox no longer supports non digitally signed plug-ins.
This was done to protect us as the consumer from hacking etc.
This problem also appears to surface when we get a JAVA update and we have to repeat this fix.
What I did learn is the last digitally signed Garmin Communicator Plug-in can be downloaded here .
So we need to download the Communicator Plugin 420.exe from the link above. Note where you saved it to.
You should then go to the top of Firefox and open Tools then Add-ons then Extensions then find Communicator Plugin (which will likely show disabled) then click remove. You should restart the browser if not asked.
I ask you to do this step because Communicator Plugin 421 was issued and was not digitally signed and since been removed and you can not install a lower version of any plug-in that is already installed.
Then go to where you saved Communicator Plugin 420.exe and install it.
Once this is done you will be asked to restart Firefox.
Connect your GPS and go to Geocaching.com and the Cache you want to load.
You will get a message saying NOT supported, don’t panic.
There is a message bar/line above the error message above saying Allow http://software.garmin.com to run “Garmin Communicator” that runs width of the page with this written on the left then on the right you will see this below.
I have always recommended that one loads their caches on a micro-sd card because several Garmin units have an issue of corrupting the GPX file stored on the unit itself. Oregon 300/400/500 and 550 units have this problem as well as GMap 62’s that I know of.
I have been out in the middle of no where with fellow cachers when this has happened. They are not happy.
If this happens when you use a micro-sd card all you do is removed the sd card restart the unit and wait a minute. Turn the unit off and reinstall the sd card and all should be good again.
I hope this helps you solve your Garmin Communicator Plug-in problem. I have used this fix to solve many fellow cachers issue with this.