Script on Mozilla Firefox

Here’s a weird one that I’m thinking someone in our community can discern. We have an Apple AirPort in our basement to distribute a wifi signal in our home. Kris and I both have MacBooks that we use upstairs. We both use Mozilla Firefox. BUT, when I try to use Firefox in the basement on either my old Powerbook G4 Notebook or on my new iMac, on some websites — including Beliefnet — it says that there is a script that mozilla does not recognize (or something like that) and it could make the computer real slow and it asks if I’d to abort the script. I always click that and then, as long as I stay on that page, it works fine. But, if I change one page, back comes the dialogue box. (1) What is this about and (2) why only in the basement?