I've used firefox for years, and love it. However, I'm not aware of firefox making an operating system (like windowsXP) -- it's an internet browser.
It's free because it's "open source," developed essentially by volunteers who wanted an alternative to Explorer.
Advantages --
It's reputed to have better security, since it's more tightly programmed, and since its smaller user base makes it a less lucrative target for hackers.
it's smaller, lighter, and much more flexible. There are a wide varieties of "skins" you can download to customize the appearance, and there are individual "extensions" that add features you like. That's a big difference from explorer, which tries to do everything for everyone. For example, I use ectensions that link to the national weather service and put the temp and forecast on the browser bar; I use a script blocking extension that lets me choose when a site gets to run scripts at all; I use a translation extension; and I use a search function extension that lets me use the same search box for everything from dictionary searches to Wiki searches and plain old google ones.
Disadvantages --
There are some pages that dont work quite right when opened by Firefox.
They also make a Outlook competitor (also for free) called Thunderbird. Used it for years too.
