3.2 for Ubuntu
Aaron Boodman

Greasemonkey is a Firefox extension that lets you use JavaScript code on certain web pages so that you can add new features to those pages.

There are many web pages using JavaScript that, when browsing, it can easily manage pages and provide a better user experience. Greasemonkey enjoys this technology to add and modify content on the page.

All these scripts are done using, which you can install like extensions. However, you can correct them inside and even, as they are just javascript code.

So, if Firefox extensions are not enough to solve all your concerns then Greasemonkey may be the solution you're looking for.

More info

License Version Developer
Free 3.2 for Ubuntu Aaron Boodman