What Greasemonkey is for developers is Customize Your Web for non-developers. You can remove/rearrange /modify elements, add keyboard shortcuts and do many other things.
The basic principle is as follows: If you want to customize a website you switch to an edit mode (either by pressing F10 or by clicking on the statusbar icon) and create a "script" consisting of a series of actions and a defintion for which sites the script should be applied. The script with its actions is then executed after every load of the website. Everything is done WYSIWYG editor (What you see is what you get).
The scripts can be edited and modified at any time by just toggling again into the edit mode.
Customization capabilities include
* Removal and rearrangement of elements (cut / copy / paste behavior)
* Modifying of elements in every possible way (changing size, color, default values etc.)
* Definition of keyboard shortcuts / shortstrings for any clickable element (shortstrings are multi-character shortcuts, e.g. "SO" for a "Sign out" Button)
* Adding keyboard navigation support for any list of elements (e.g. for the result entries of search)
* Definition of small macros which are triggered by a shortcut
* Automatically triggering actions on page load (e.g. for automatic click on a login button or focusing a special field)
- Minor bugfix for Firefox 20