By default, Firefox 2 supports only three web feed readers: Bloglines, Yahoo! and Google Reader. However, you can add as many providers as you wish and use any of them as your subscription manager.

You will need to tweak about:config and add some preferences manuall. 

1. Enter about:config in the location bar
2. Create a new preference: Right-click on any preference and select New>String
3. Enter browser.contentHandlers.types.6.title form the preference name. Then enter Netvibes for the value. Note that as you add more web feeds reader you will need to change the 6 to the immediately next number. Since Firefox 2 comes with 3 web feed readers plus other three for internal purposes, 6 is the first available.
4. Create a new preference and enter browser.contentHandlers.types.6.type for the preference name. Enter application/vnd.mozilla.maybe.feed for the value.
5. Create a new preference name and enter browser.contentHandlers.types.6.uri for the preference name. Enter http://www.netvibes.com/subscribe.php?url=%s for the preferencevalue.
6. Restart Firefox to see the effect.