Version 2 of the Google Firefox toolbar has a "Subscribe to Feed" feature that automatically detects if the webpage offers XML feeds (much like Firefox LiveBookmarks or Windows Live Toolbar). When you click "Subscribe", Google Toolbar takes to your chosen feed reader to confirm the subscription. XML Buttons and Feed services like Solosub or MultiRSS could soon run out of business.
Google Toolbar 2 offers tight integration with GMail - When you click the mailto: hyperlink on a webpage, GMail Compose Message screen will automatically open. Also, the Google Safe browsing extension is now integrated into the Google Toolbar.
The default Google Search is also replaced with a combination of Google Suggest, Toolbar history and your own Bookmarks.
More release information on Google Firefox Toolbar 2.0 here and here.