Firefox JSTerm v2 is available.

JSTerm:

Changelog:

JSTerm is an experimental restartless addon. It adds a "jsterm" button in the developer Toolbar (Firefox 16+). The terminal behaves like a shell terminal (history, completion on tab, key bindings, …). It's a classic REPL environnement. It uses the builtin Firefox Source Editor to highlight JavaScript. It uses a Sandbox to eval JavaScript. And it comes with an Object Inspector.

For Firefox and Addons developers: like Scratchpad, this tool comes with a "chrome" mode (type ":chrome" to enable it).