They both have add-ons and themes, tabbed-browsing and a comprehensive download manager. But these are the things they do different, and the reasons I'm keeping both on my computer:

Google Chrome:

• More lightweight, crashes less.
• No status bar at the bottom (I used to dislike this, but actually it's kind of convenient).
• Navigating through my Tumblr dashboard is actually more convenient here than on Firefox.
• Awesome New Tab startpage with thumbnails.
• Doesn't take ages to open.

Firefox:

• While updating my Livejournal using the Rich Text editor I can toggle bold/italic/underlined format with keyboard shortcuts (can't do it on Chrome).
• The bookmarks toolbar stays there all the time if I want it (whereas I have to press Ctrl+B for it to appear on Chrome if I'm not on the New Tab page). This is convenient especially for certain bookmarklets and dictionary pages I use often.
• The Del.Icio.Us add-on works better (seamless integration, no new tabs necessary).
• ChatZilla means I need no separate IRC client.

And possibly more, but that's it for now.