I'm not sure about the wisdom of adding new features before the 1.0 release of Firefox, but I have to say that the two new features being added right now are pretty awesome.
The first new feature isn't technically new; it's just an improved UI for an old, beloved feature. The Find-As-You-Type (FAYT) feature has gotten a new toolbar that pops up. This gives a nice UI that will make what's going on more clear to new users. It also features the ability to choose to "highlight" the search term wherever it appears in the current document. This is a feature I've wanted for a long time, so I'm very happy to see it.
The default way to trigger this functionality has also changed. Prior to 1.0, FAYT worked on links only by default, and was triggered by simply typing (with the '/' character used to do full-text searching). I always switched it over to full-text searching when I started my profile anew. The new functionality is to do full-text searching by default, but require either the '/' character to trigger it, or Ctrl+f.
The other new functionality is being called "LiveMarks" right now. It's basically a way to integrate RSS into the browser. Special bookmark folder for RSS feeds can be created when you go to a page. I assume that it relies on tags on the page that allow feed auto-discovery. You use the Feed bookmark creation tool at that appears in the status-bar, and bookmark the feed. A folder is made for the feed you are bookmarking. Then, when going to the that folder in your bookmarks, all the posts from the feed are shown as bookmarks. I'm writing this post partially to check out how it indicates new posts in the feed.
My description of these features may not do them justice. So check out some screen-shots, or try out the features by using a recent build. Be prepared for bugginess, as these changes were checked-in recently, and are rapidly being worked on and cleaned up.