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.

https://www.google.com/earth/explore/products/plugin.html

More information about it below:

https://support.mozilla.org/en-US/kb/why-do-i-have-click-activate-plugins?as=u&utm_source=inproduct

Other than that, starting Google Earth should be done from the standard Start Menu location.

Hope this helps.

Grif