Firebug has a console (the first tab of the firebug window) where you can write JS code and execute it in the current page. Suppose the webpage has a function called myFunc
defined and you want to override it: executing the following in the console will do the trick.
myFunc = function(/* function arguments here */) {
/* new function body here */
}; /* notice the ; */
and to see the the current version of a function you can execute
uneval(myFunc);
this will print out the current myFunc
note: I think that by default the console comes with a single-line input box, but somewhere in the options you can switch to a multi-line input box. When in multi-line, goes to a new line, + executes the code.