Both Firefox and Internet Explorer "auto-discover" Web feeds if the website provides the necessary clues (hidden links).

Firefox: The auto-discovery icon shows up at the right end of the address (URL) bar. Clicking the icon presents the web feed choices.

Internet Explorer: The auto-discovery icon located in the tab bar (IE7 or IE8) "lights up". Clicking the down arrow presents the web feed choices.
If no auto-discover is provided (sometimes there's a good reason) you, the user, must discover the feeds yourself. ;-)
Look for an the RSS badge:
,
or
. If you don't find one there may be a plain "RSS" or Feeds link somewhere on the website. In the simple case, just click one of them and follow directions. In the not-so-simple case, see the Firefox or IE7 instructions below. :-)
See more about discovering feeds and subscribing at the Web feed introduction page.
When you click a web feed icon, firefox gives you a few built-in options, plus the option to "Choose Application...". The latter one opens a file selection box, and you navigate to any stand-alone feed reader you want to use.
If you're using a Firefox add-on that supports Live Bookmarks, e.g. Brief, Feed Sidebar, NewsFox, just pick the Live Bookmarks option.
If you're using a web-based or stand-alone reader that is not shown, it may be easier to copy the link from the icon [right-click > "Copy Link Location"] and paste it in the subscription box of your feed reader.
If you go to Live Bookmarks in Firefox Help & Tutorials you'll find a lot of useful information there too. ;-)

There are not many feed-reader add-ons for IE7, and I have not evaluated any of them except Feed Plus, which adds very little. The best way that I've found to extend IE7 is to install Windows Live Mail — yes, the email program — or RSS Bandit.
Both of those programs automatically tie in with IE7 feed subscriptions through the "Windows RSS Platform". The article may leave the impression that this only works in Vista, but it works with Windows XP if you have IE7 and SP3 installed.
If you're using a web-based or stand-alone reader that is not shown, it may be easier to copy the link from the icon [right-click > "Copy Shortcut"] and paste it in the subscription box of your feed reader.
If you look up "Using feeds (RSS)" in IE7 Help, you'll find a lot of useful information there too. ;-)
That's what I do with my Comics.com feed because it works much better than the feed itself. Don't use my feed though, get one for yourself with your own favorite comics. :-)
"You do not really understand something unless you can explain it to your grandmother." ~ Albert Einstein