How do I execute a page-defined JavaScript function from a Firefox extension?

document.wrappedJSObject.funcToBeCalled();

This is not secure and allows a malicious page to elevate its permissions to those of your extension... But, it does do what you asked. Read up on the early greasemonkey vulnerabilities for why this is a bad idea.

It is for security reasons that you have limited access to the content page from extension. See XPCNativeWrapper and Safely accessing content DOM from chrome,

If you control the page, the best way to do this is set up an event listener in the page and dispatch an event from your extension (addEventListener in the page, dispatchEvent in the extension).

Otherwise, see http://groups.google.com/group/mozilla.dev.extensions/msg/bdf1de5fb305d365

I have a very simpler way to do it. Suppose you have to call xyz() function which is written on page. and you have to call it from your pluggin.

create a button ("make it invisible. so it wont disturb your page"). on onclick of that button call this xyz() function.

<input type="button" id="testbutton" onclick="xyz()" />

now in pluggin you have a document object for the page. suppose its mainDoc

where you want to call xyz(), just execute this line

mainDoc.getElementById('testbutton').click();

it will call the xyz() function.

good luck :)