Talvez você possa usar a extensão keyconfig para isso?
Neste link , algumas pessoas dizem que o fizeram funcionar no Firefox 3:
@ M.C yes it is !!! (compatible with Firefox 3.6) you have to extract the file "install.rdf" from "functions_for_keyconfig-1.4.0.xpi" with a deziper prog like "WinZip" in Windows or "file roller" in Linux.
Edit "install.rdf" and search for this line: "3.5.". Then replace with "3.6.".
In "functions_for_keyconfig-1.4.0.xpi" replace file "install.rdf" with the modified one. Save. Open your Firefox 3.6, drag and drop the file "functions_for_keyconfig-1.4.0.xpi" in Firefox windows. That's it.