This add-on allows you to enter your location using longitude and latitude (available from http://www.whatismygeolocation.com/
for free) for use with geolocation enabled websites.
Firefox 3.1 will eventually feature built-in geolocation providers, likely including some manual solution such as this one. To use it, simply install, look up your location's co-ordinates, then enter them by going to Tools > Add-Ons > Geolocation > Preferences. Whenever a website asks for your location (using the W3C Geolocation API) you will be asked if you want to provide it.