Is it possible to read the clipboard in Firefox, Safari and Chrome using JavaScript?

Safari supports reading the clipboard during onpaste events:

Information

You want to do something like:

someDomNode.onpaste = function(e) {
    var paste = e.clipboardData && e.clipboardData.getData ?
        e.clipboardData.getData('text/plain') :                // Standard
        window.clipboardData && window.clipboardData.getData ?
        window.clipboardData.getData('Text') :                 // MS
        false;
    if(paste) {
        // ...
    }
};

Online Spreadsheets hook Ctrl+C, Ctrl+V events and transfer focus to a hidden TextArea control and either set it contents to desired new clipboard contents for copy or read its contents after the event had finished for paste.

NO. And if you do find a hack (e.g. old version of flash) do not depend on it.

Can I ask why you want to read from the clipboard? If the user wants to pass along the clipboard contents, all they need to do is paste.

I believe people use a hidden Flash element to read the clipboard data from the browsers you mentioned.