Java Applet: Java Version, update

I am making a Java Applet right now. I have compiled with Java 1.5.

When I run my applet in Firefox it does not run. When I open the console in the browser (right click 开发者_高级运维on the applet -> view console) it tells me that I am only running Java 1.4.2. In my browser.

1. I am wondering is it safe to develop my app for 1.5 and assume that everyone will have that version?
2. How do I update the version of Java Firefox is using? I looked in preferences but I did not see anything (I am using Mac OS X, 10.4.11).

I am not sure if this question would be better suited to super user.

I'm not sure for Mac OS X 10.4, but there is a "Java Preferences" application in /Applications/Utilities/ that allows to change the version used in java applets (picture at the end).

And is it safe to develop for 1.5, let just say that Java 1.5 isn't supported anymore by Sun/Oracle since November 3, 2009 (almost a year). So it really depends on you and your clients/users, if they still use Java 1.4 for some specific reasons you know about (company policy, etc.) you should make it work with java 1.4.