The new version of Firefox 2.0 introduces a new and simple way to integrate Firefox with aggregators. While you needed an extra extension to add subscriptions from Firefox 1.5 to Liferea, now you can add Liferea support without any extra software.
You can now tell Firefox 2.0 to use the "liferea-add-feed" script to add new subscriptions to Liferea. You can change the feed handling either by going to the Preferences dialog or by clicking on the feed symbol. In both cases you will be presented with a menu where you can select "Choose Feed Reader". After clicking a file selection should popup. Select "/usr/bin/liferea-add-feed" and you are done!
Note: If you were using FeedBag with Firefox 1.5 it should have been disabled automatically and you can safely remove the extension.