*** Firefox 3 guide can be found here. ***

Customizing Firefox 4 is as simple as the old version. Most steps are the same but lets start from beginning:

1. Picking up gtk theme, which will make Firefox native in KDE.
Hugo's oxygen-gtk is still the best choise. Its available at GIT but some distributions are already using it as default theme (openSUSE, Kubuntu). Package for Arch Linux is available in [extra].

2. Sorting out Firefox's theme.
Oxygen KDE looks flawlessly amazing. Pick up version 3.0, which is compatible with Firefox 4 and restart your browser. Now right click under your titlebar, on menubar (if you have) and uncheck "Menu bar" option. You'll see the new Firefox button menu, just in front of browser tabs.
Some additional options are also available with Oxygen KDE theme. You can check them with right clicking somewhere around address/navigation bar and selecting Oxygen KDE options.

3. Changing the appearance of the file selector.
Type about:config in your address bar and search for ui.allow_platform_file_picker. Set it to false.

You are done! Congratulations!