One of the great things about Firefox is tabbed browsing. Tabbed browsing is not limited to Firefox, but is primarily a Mozillaesque thing, and I will be ignoring other browsers in what follows.
Tabbed browsing comes out of the box in Firefox, complete with numerous options, but many extra features for tabs are available through extensions. Many features of tabbed browsing (and other favourites) have been popularised in extensions and then added to the base code. These extensions are quite popular, so some of what I describe here and is "extension only" will probably be standard in a close release. You can use the browser without ever using tabs -- just separate browser windows -- but you'll be missing out on a whole lot of fun.
Tabs are one of a few MDI looks. Historically, most MDIs have been awful, but this is one that really works. Basically, a single browser window may "contain" several different webpages (or any other browsable goodies). Only one is actually displayed, but a tab-bar with named and iconed tabs for each page is displayed along the top (or bottom) of the main content (non-chrome) box. Selecting a different tab shifts focus to that page. Middle clicking a link opens that link in a new tab on the same window. Additionally, you can set various window-opening events (javascript-induced, external applications, popups, and so on) to actually only open a new tab. The tab-bar may also be used (depending on tabbing extensions and the preferences you set) to remove a tab, open a new (blank or homepage) tab, restore a deleted tab, reorder tabs, reload a tab (without even viewing it), and much more. This means it not only keeps your currently browsed pages separate from other windows (as, e.g., when you simply have several browser windows on your Windows or Windows-like taskbar), but adds specifically browser-related functionality. The tabs themselves may also display useful information, such as whether the contents are unread.
I usually have several pages I'm about to read (and wouldn't like to forget about just yet), or want to have on hand as a reference (e.g. when noding about something I need to read up on). I used to only ever open two or three web pages at once: the clutter was just too much to handle. Now I easily go to a dozen if I'm skimming Slashdot, reading or writing e2, or even just grappling with a surge of correspondence on Gmail. It's also possible to bookmark your set of related tabs all at once, and converesly to open all bookmarks in a bookmark folder simultaneously, in tabs. Tabs are a great way to instil some order and generally, well, keep tabs on things.