When you visit a website which requires the plugin to activate it will. When such is needed, you'll be requested by Firefox to activate the plugin, it that's why you want. Remember there can be security issues with such add-ons. As an example, activate the plugin at the link below.
More information about it below:
Other than that, starting Google Earth should be done from the standard Start Menu location.
Hope this helps.
I have in my Firefox add-ons a Pugin for Google Earth. I am puzzled how to launch it next, for I don't see an icon for it on my desk top.
The answer to this query must be very simple!