The primary advantage to Firefox is the Gecko rendering engine - full W3C standards compliance, a 'quirks' mode for rendering poorly written pages, and full JavaScript support. It's likea combination of Netsurf and Oregano. Also, unlike Oregano, it's designed for the desktop, so it's more intuitive in that sense.
Other feautres are tabbed browsing, which you really have to use to understand how good it is. There's also industry standard plugin suuport (so you can get Flash, Shockwave, etc. assuming these will work on RISC OS of course) and it's exntentions and themes. The extensions make it highly customisable. It's difficult to not find an extension you're looking for, to fulfill a certain task. Also, most of these are platform independant, as they're coded in xhtml/html/javascript.
Or, you could always look here for more info: [link]