This is the extension built by Chad Smith. I have just updated it to work with the latest versions of Firefox. This addon allows you to use your Google Voice number to call phone numbers in website pages using a click-2-call feature. Only works in firefox version 4 and under.
Once downloaded, open it with firefox by going to File>Open
Also, “Like” this site using the link below to stay updated when a newer version is ready.
Dr. Elijah Clark (July 17, 2014). Google Voice – Firefox 4.0 Extension Addon [Web log post]. Retrieved from http://elijahclark.com/google-voice-firefox-40-extension-addon/