KDE's scroll bar, as you can see to the right, is a bit unique. What seperates it from many is the up and down scroll buttons on the bottom of the scroll bar. Some people seem to like this more than others. I like to use these scroll buttons to scroll when reading long documents myself, though I'm not quite sure why. Thankfully, the KDE developers made this easy to add to Firefox and Thunderbird.

1. Open up the KDE Control Center. If it is not on your distro's menu, then can press Alt+F2 and then type in the command kcontrol.
2. Navigate to Appearance & Themes and then GTK Styles and Fonts.

3. Under the last section, which is likely marked with a Firefox logo, click to install the scrollbar fix.

4.You should then get a notification dialog to restart Firefox and Thunderbird to see the effects. Once restarted, you should now see the KDE-like scroll button setup in these applications as well.

The Gentoo wiki has a nice page on more ways to integrate Firefox Firefox with KDE, most of which work on any distro. Konquefox is another great resource for this.