If you'd prefer your CPU cycles be spent rendering web pages instead of animating tabs, do this in Firefox 4:

1. Type "about:config" into your address bar, then hit Enter
2. In the "Filter" field, type "browser.tabs.animate" or any substring thereof
3. Set the value for for the matching "Preference Name" that comes up to "false" by double clicking the "Value" column
4. Close the about:config tab

And that's it. Screenshot below:

The sheer number of settings customization in Firefox continues to be one of the biggest advantages of using it.