Fuzz testing or fuzzing is a software testing technique that provides random data ("fuzz") to the inputs of a program. If the program fails (for example, by crashing, or by failing built-in code assertions), the defects can be noted. Wikipedia
The files attached to the JSFunFuzz tickets that announced the project, also provides implementation for in-browser fuzzing although it seams to be neither funny nor very useful. Most of the time, the CPU load is around 100% and the user gets some low frequently updated text output which spits out info about the concatenated strings and the debug output they provoked. Nevertheless some people already implemented solutions similar to this one. Do you remember that CPU load will probably rise sky high if you click this link?
The question is how to use the JSFunFuzz fuzzer at home. Quite easy, I must say. Just grab a copy of the Rhino engine from Mozilla. Install a JRE if you do not have one yet, and use your console to instantiate Rhino. Something like the following should do the job:
java -jar /path/to/js.jar
java -jar /path/to/js.jar /path/to/jsparsefuzz.js