Firefox 28 - Communicator Plugin Not Found - but only on GarminConnect...

Check if your plugin is "Always activate" in firefox. If it's set "By default" It can be your problem.

Hope this can help

The reason for this is that Garmin needs to update their javascript code - the way they detect the plugin is broken! Starting with Firefox 28 Mozilla introduced a privacy feature that websites are no longer allowed to ask for all installed plugins, they need to request the plugin directly.
Technical details: https://developer.mozilla.org/en-US/docs/Web/API/NavigatorPlugins.plugins

THIS DOES NOT WORK IN FIREFOX 28 OR LATER:
Javascript code from: http://connect.garmin.com
detectPlugin: function() {
// allow for multiple checks in a single pass
var daPlugins = PluginDetect.detectPlugin.arguments;
// consider pluginFound to be false until proven true
var pluginFound = false;
// if plugins array is there and not fake
if (navigator.plugins && navigator.plugins.length > 0) {
var pluginsArrayLength = navigator.plugins.length;
// for each plugin...
for (pluginsArrayCounter=0; pluginsArrayCounter < pluginsArrayLength; pluginsArrayCounter++ ) {
// loop through all desired names and check each against the current plugin name
var numFound = 0;
for(namesCounter=0; namesCounter < daPlugins.length; namesCounter++) {
// if desired plugin name is found in either plugin name or description
if( (navigator.plugins[pluginsArrayCounter].name.indexOf(daPlugins[namesCounter]) >= 0) ||
(navigator.plugins[pluginsArrayCounter].description.indexOf(daPlugins[namesCounter]) >= 0) ) {
// this name was found
numFound++;
}
}
// now that we have checked all the required names against this one plugin,
// if the number we found matches the total number provided then we were successful
if(numFound == daPlugins.length) {
pluginFound = true;
// if we've found the plugin, we can stop looking through at the rest of the plugins
break;
}
}
}
return pluginFound;
}


THIS WORKS IN FIREFOX 28 AND LATER:
Javascript code from: http://software.garmin.com/en-US/gcp.html

var pluginFound = PluginDetect.detectPluginByMIME('application/vnd-garmin.mygarmin');
[...]
detectPluginByMIME: function(mimeTypeName) {
if (navigator.mimeTypes) {
var mimeType = navigator.mimeTypes[mimeTypeName];
return !!(mimeType && mimeType.enabledPlugin);
}
else {
return false;
}
},


I was wondering why my Firefox on Linux behaves differently than Firefox on Windows. On Windows it worked in Firefox 29, on Linux it did not.
So apparently you are able to configure the new privacy feature and restore the old behavior. And apparently Windows Users that download from Mozilla do not need any privacy, since the new feature is disabled there.

Go to about:config search the variable plugins.enumerable_names and change the value to * or even better add "Garmin Communicator"

Example:
plugins.enumerable_names = Java,Nexus Personal,QuickTime,Shockwave,Garmin Communicator