Greasemonkey is a Mozilla Firefox extension that allows users to install scripts that make on-the-fly changes to web page content after or before the page is loaded in the browser (also known as augmented browsing).
The changes made to the web pages are executed every time the page is
viewed, making them effectively permanent for the user running the
script.
Greasemonkey can be used for customizing page appearance, adding new
functions to web pages (for example, embedding price comparisons within
shopping sites), fixing rendering bugs, combining data from multiple web
pages, and numerous other purposes.
Greasemonkey is available for Firefox, Flock and Epiphany. The Epiphany Greasemonkey extension is part of the Epiphany-extensions package. However, this extension is not fully compatible as of release 2.15.1, since some Greasemonkey API functions (e.g. GM_getValue) are unsupported. There are also custom versions for SeaMonkey[9] and Songbird.[10]