Another Chrome-inspired feature in Firefox 4 is the ability to "pin" Web apps to the tab bar. Open up any Web app (or any Web page, really) in a new tab, right click the tab, then select "Pin as App Tab." This will add a tab showing only that page's icon to the far left edge of the tab bar.

When you close then re-open Firefox, the pinned tabs will remain there. Whenever something in the Web app you pin updates--say, you get a new message in your Gmail inbox--the pinned tab will turn blue to notify you (again, very similar to Google Chrome).

These additions are great if you use lots of Web apps, but they're features that Chrome already has, so it does feel a bit like Firefox is playing catch-up here.