Bug fixes vis-a-vis current Firefox

The ua.version comparison needs to be a float comparison, not string. (I.e., because current FF versions like "18.0" compare as < "2.0").

I also removed the explicit "font-size: 100px" setting because, in my own tests, that broke the element substitution in Firefox. The clickable area never matched the size of the replacement element unless I removed that font-size.