Well this is simply a case of the Tiscali developers not following the W3C standards for web development, and they should know better. The problem is not Firefox, as Firefox adheres to these standards, it is simply the website is using certain technologies that only render correctly in IE, and that is simply a case of Tiscali rushing out website updates without performing cross-browser testing, or simply not caring as IE still dominates the market.
Sorry rant over, I am not a big fan of Tiscali, anyway, there is a great 'fix' for this kind of problem (websites that don�t render correctly in Firefox), simply install the IE Tab extension, it basically uses the same engine that IE does, just wraps it up within Firefox. And when you see a site (as in pretty much all Microsoft�s own sites, and in this case Tiscali�s) that does not look quite right, you simply click a button and it uses the IE engine, but still within Firefox, and better still, you can then tell it to always use the IE engine for this site so that next time you visit it, all is well.
Hope this helps.
Paul