If you are not noticing this oddity on the same computer screen when using a different browser, such as IE or Google Chrome then the problem is not related to your graphics card, drivers or any hardware - such as a bad mouse or dirty mouse causing jerky cursor movements - but to a corrupt install of Firefox.... I presume you are using the latest version of FF which is version 3.5.4 ....
If not using this more secure version, download and save the Firefox installation file (
http://www.mozilla.com/en-US/firefox/personal.html ), then go into Control Panel and Add/Remove so you can uninstall Firefox.... you will be offered the chance to save settings and add-ons and you should do so.... Once the uninstall of FF 3.5.3 or whatever version you are using is done, go back to your saved download and double click it to quickly reinstall Firefox 3.5.4 which will use your saved settings from the uninstall you did.... I found that all of my add-ons/extensions from the earlier version worked just fine with the newest version of Firefox... I have had several of those add-ons even update just in the last couple of days...
There is a beta version of FF, called 3.6 which is out but many problems are occurring with it while it is in this beta version, so just get the latest finalized version of FF, 3.5.4