Depending on your ISP, I would suggest looking at their support page or contacting them direct for possible help.
The reason why I say that, is due to me having a similar problem, which was resolved by the ISP via their instructions regarding changing things on the computer set-up. Took about 20/30 minutes, but worth it in the end.
Hi, Jock1e, I posted a link to the ipconfig results. I run a holiday firm website and had to put the screen print somewhere as this forum doesn't allow it. The link that says ipconfig results should work....I'll try again.
ITB, as bumpkin has already stated, your dad's IP configuration looks fine.
I've tackled that pesky Firefox XPCOM a couple of times and I didn't need to uninstall anything but simply installed over the top of the existing version. Use a working computer and download the Firefox setup file then save it to a memory stick - here's the direct link to the British version.