The new version of Firefox, namely Firefox 2.0 Beta 1, takes most of its new features from Google Toolbar for Firefox. And that's no surprise as the lead Firefox engineer, Ben Goodger, works at Google.
* Google Suggest in the search box. The Firefox implementation doesn't include your bookmarks and the results from the search history, but it's basically the same feature.
* Anti-phishing protection. Although not enabled by default, Firefox uses the same Google Safe Browsing technology from Google Toolbar. Previously that was a standalone extension. "By combining advanced algorithms with reports about misleading pages from a number of sources, Safe Browsing is often able to automatically warn you when you encounter a page that's trying to trick you into disclosing personal information."
* Subscribe to feeds. Like in Google Toolbar, you can subscribe to feeds using Live Bookmarks or a feed reader like Bloglines.
* Spell checking. The differences between Google Toolbar's SpellCheck and Firefox's new feature are that in Firefox every misspelled word is underlined automatically and Firefox uses a local dictionary, while Google queries their server. Google's dictionary is much more comprehensive and it's dynamically updated, but Firefox makes the feature more useful by integrating it in the normal use.
With all these features included in Firefox 2.0 by default, there's little reason to install Google Toolbar. I wonder if this is the final aim of any plug-in / extension: being integrated in the application by default?