If you have a Thinkpad, and you have those grey page-buttons around your arrow keys, this is what you do to make them work as Forward/Backward in Firefox 2.

1. You will have to find the directory in which Firefox is installed. On a standard Kubuntu Feisty install, it’s in /usr/share/firefox.
2. Edit <firefox-directory>/chrome/browser/content/browser/browser.xul, and find the line that says <keyset id="mainKeyset">.
3. Paste the following lines directly underneath:

<key id="goBackKb" keycode="VK_F19" command="Browser:Back" />
<key id="goForwardKb" keycode="VK_F20" command="Browser:Forward" />

4. Save and restart Firefox.

Now you can use the page-buttons to move back and forth through your Firefox history. This also works in Firefox 1.5, but requires a different approach. You can read more at the ThinkWiki.