Ok, here's how to add it to <a href="http://www.mozilla.org/products/firefox/">Firefox</a>:
Click "Bookmarks -> Manage Bookmarks"
Click "File -> New
Live Bookmark"
In the properties window that pops up, give it whatever name you want in the "Name:" field (like Mp3car or something).
In the "Feed Location" field, paste:
http://www.mp3car.com/vbulletin/external.php?type=rss
In the Description Field put: "I will name my first born child Chris" or something similarly suitable.
Hit Ok to add the RSS feed to your bookmarks.
Then, from the Firefox Bookmarks Manager (the screen that shows your bookmarks and the new MP3Car feed), drag the new MP3Car bookmark link to your Firefox toolbar somewhere.
Viola! Now your browser will update the RSS feed automatically. You can just click the orange/white drop down box to see the forum posts.
Cheers,
-CB
P.S.
If you're nasty you can even click the "Open in Tabs" option of the feed for some great mp3car.com action! (Seriously, this puts some load on the server so don't do this too often). Have fun!