Comment 7 for bug 1198490

Hmm, there may be a bug in the script - but I thought that I'd pretty well tested that. Anyway, you can download the file manually using this command:
wget -O FirefoxSetup.exe http://download.mozilla.org/?product=firefox-18.0.1&os=win&lang=en-US
and then you can move it to that folder by using:
sudo mv FirefoxSetup.exe /usr/share/wine-browser-installer/FirefoxSetup.exe

That will likely do the trick for you.