The firefox add-on Greasemonkey enables the firefox user to modify web content locally. It does this by running a user javascript file after the page has been rendered. The user script has to be installed and the pages for which it is intended have to be defined (this can also be done in the script file).
Tutorial
-Install the add-on Greasemonkey.
-create the javascript file, but name it *.user.js
-make sure the greasemonkey addon is active (the icon on the browsers lower right corner is in color).
-right-click on the icon (or click Extras->Greasemonkey), install and activate the script and define the pages after which the script should be run.
-open a page for which the script was written and observe the effect!
There is a repository for greasemonkey scripts here
P.S.: You can also install the script by simply opening it with firefox.