Question

This piece of code doesn’t work on my firefox 12 browser.

javascript:alert("hi");

But it works on IE8.

What’s the problem?

Answer 1

see http://support.mozilla.org/en-US/questions/876916#answer-248299

You can no longer run JavaScript code via the location bar in Firefox
6 and later. You can see an error message in the Tools > Error
Console. That code now gets a null principal for security reasons and
doesn’t have any effect (Bug 656433). You need to create a (keyword)
bookmarklet and run the code by invoking that bookmark.

NoScript can allow you to run such code via the location bar, see:

http://forums.informaction.com/viewtopic.php?f=18&t=6488&p=27991

Google chrome has a different approach, if you copypaste javascript: to your location bar, it will be silently swallowed. You can only run it if you directly write it out in the location bar.

Both browsers come with a built-in console that can be used to run code that affects the current page.