I noticed these nice firefox 3 features haphazardly :
On the tab bar : 
you can duplicate a tab thanks to a ctrl+click
you can move the tabs left or right using your mouse scrolling button

By the way, one thing I do not understand for Firefox is why open new tab does not open your start page ?

1 trick for KDE
Right click on a window name bar and you have the option to keep the window above others. That is nice to do some copy/paste from one page to a notepad or also to keep the command line while typing code in an IDE….
 

In the same kind of ideas as my post on 2 monitors, it would be great to be able to tile windows under Gnome or KDE. Some people thought about it as some softs do it : whaw or a compiz plugin (but using compiz to do that is really an overkill).