XPS is Microsoft’s version of Adobe’s PDF. Its major advantage is that it is preinstalled on Vista and XP SP3. You can easy print everything as an XPS for later viewing. But Firefox users might encounter a weird bug when they try to open a XPS file. When a XPS is opened, Firefox starts and asks if you want to download or open this file. If you click download, you can save it somewhere and if you click open, Firefox opens a new tab and starts asking the same question again. Why is Firefox the default viewer for XPS if can’t open them?
The reason is simple: Microsoft Internet Explorer is the built-in XPS viewer, but Firefox takes all Internet Explorer file assosiations when installed as the default browser. So all you need to do for viewing a XPS file is to right click on it, select “Open with…” and choose Internet Explorer. You can make it the default application for XPS files too, then you don’t need to worry again when you open a XPS the next time.