The plugin also gets blocked by default, so you have to choose "allow and remember" to keep it active. I got a pop-up asking me to activate Java when I first ran RS via Waterfox.
I got the version from here: http://www.java.com/en/download/manual.jsp (Windows 7 offline 64-bit)
Apparently some people had to install that version to correct the client launching error.
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Whether or not you notice a difference depends on the system I guess. Some had good results and others had worse performance. For me, it improved at least.
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Yeah if you have 64 bit java, it should just straight away move you into the browser game instead of automatically bringing up the client. You'll have to allow and blah blah blah then but it should work then