UPDATE: Many readers have confirmed that this trick also works in previous versions.
It seems Mozilla team is really hard working on Firefox's upcoming version 4.0.
A new sweet animation has been added to "Preferences" (Options) window which is currently disabled by default but can be enabled using "about:config" method.
To enable this new animation you can use following simple steps:
1. Open Firefox and type about:config in the addressbar and press Enter. It'll confirm, click on I'll be careful, I promise! button.
2. Now type animatefade in the Filter box and look for following boolean key:
3. The entry "browser.preferences.animateFadeIn" would be set to false. Double-click on it or right-click on it and select "Toggle". It'll set its value to true.
4. Now select "Tools -> Options" and navigate through various tabs and you'll see the new animation while changing to a tab.
The window length will increase/decrease according to the number of options available in the current tab as shown in following video:
PS: I can remember the feature was first seen in Safari web browser.