If you just do it on your own computer, the packet will be already dropped by your own gateway. You can fake whichever address in your local subnet, but those are very likely remapped anyway in your gw to the one given by your ISP.

If you would have access to the switch port used by your ISP in the Internet exchange point (IX), you would have more liberties in choosing the IP.

Did you give it to it?

It can be a pretty nice feature for using map-based apps in the browser.

I haven’t used such websites for a while and I don’t see Firefox in the recent users of the location API, even though I use Firefox Android all the time. (Info available in Android under Settings/Location.)