If you can use the torrent do so, but if you're less technically-inclined or on limited bandwidth just right-click and download the exe. If you have an older preview or pre-1.0 version of Firefox installed, I suggest exporting a copy of your bookmarks for peace of mind before installing. Uninstalling the previous version before installing 1.0 is recommended, but I didn't bother; just tweaked some values in about:config to carry over my extensions (all apart from SessionSaver took this okay.) At a guess, it'll be a day or so before update.mozilla.org is responsive enough to allow downloading extensions; like I said, millions of downloads will be battering the server.
With the release of 1.0, now is an excellent time to prod those family members and friends who think that the internet is a big blue 'e' icon to choose a browser that will make their time online safer and more productive. The Firefox developers have striven to provide an intuitive, clutter-free interface for beginners (whilst more confident users have the option to use a wide variety of powerful extensions if they wish.)
Reasons to choose Firefox/Mozilla:
Security. Internet Explorer is fundamentally insecure—how many times have people complained to you about their computer running slowly, their start page mysteriously changing or strange toolbars popping up? Firefox has several mechanisms to prevent websites automatically installing annoyances and spyware/viruses like this.
Features. There's a built-in popup blocker. You can easily install an ad-blocking extension. There's a highly useful "find" toolbar. Pages can be opened in tabs, so that you have less clutter (and you can open a bookmark folder directly into tabs!)
Interface. I've mentioned some practical things under 'features', but Firefox also allows you to choose different themes for icons and the rest of its interface. Not something I care about personally, but many people find it fun.
Standards. Many sites increasingly render poorly in older versions of Internet Explorer, as it wasn't until version 6.0 that Microsoft started supporting web standards more. IE hasn't appreciably changed in years, and Microsoft display no interest in fixing or improving it except where bad publicity costs them money.
Price. The only other major alternative browser for Windows is Opera, which is supported by adverts in its main window. Firefox is genuinely free. It's written by a large number of people who wanted a better experience of the web for themselves, who are sharing their solution. Philanthropy is not dead. :)
As an additional note: remember, if someone is using web software such as the MSN Browser, Avant Browser, MYIE2/Maxthon, Crazybrowser or NetCaptor, they're still using Internet Explorer—all these programs do is layer a different interface on top. Pages are still displayed using the same insecure and vulnerable code, so try to avoid these shells.