When Firefox opens and goes to the Java website there will be an indication on the screen that an older version of Java has been detected. There is a link on the page under the red FREE JAVA DOWNLOAD button that says "Skip installation of the current version and test the currently installed version of Java". Click this link and on the blank window click on the "Missing Plugin" link. This will install Java 6 for you.
When the install is complete quit Firefox -> Firefox / Quit Firefox. Then relaunch Firefox and enter http://java.com/en/download/testjava.jsp and click the link "test the currently installed version of Java". If Java still does not work then follow these directions below:
Click on the Mac Desktop and then in the menu above click on GO and select Utilities in the menu. find the Terminal utility. In the black window copy and paste the following lines of text one at a time. When you paste the first line you will be prompted for a password, enter your Macbook Air log on account password and press enter. Re-paste the first line of text.
Then cut and paste all the remaining lines of text into the terminal window.
sudo mkdir -p /Library/Internet\ Plug-Ins/disabled
sudo mv /Library/Internet\ Plug-Ins/JavaAppletPlugin.plugin /Library/Internet\ Plug-Ins/disabled
sudo ln -sf /System/Library/Java/Support/Deploy.bundle/Contents/Resources/JavaPlugin2_NPAPI.plugin /Library/Internet\ Plug-Ins/JavaAppletPlugin.plugin
sudo ln -sf /System/Library/Frameworks/JavaVM.framework/Commands/javaws /usr/bin/javaws
When you have entered each line of text above, close the terminal window. Then in Firefox enter http://java.com/en/download/testjava.jsp. You should be able now to activate Java by clicking "Always run on this site" at the top of the screen and see Java run.