This could lead to phishing/Trojans. All one would have to do is use the script and point it towards a modified binary. Just the other day, someone I know searched Yahoo for "firefox" and ended up clicking on the seemingly-first result - an advertisement going to an arbitrary non-Mozilla site, offering a download link hosted on their domain. Hook line sinker.
This is something that every website should have installed, the problem, is that the sites that'll actually have this installed are the same sites visited by people who have already upgraded to the latest version of firefox.
As much as I wish this were the end-all solution to getting people off of old browsers, I fear time may be the only true solution. The script is pretty neat though, worth the 10K add (or smaller compressed).