Firefox is the browser portion of SeaMonkey as a free, open source web browser developed by the Mozilla community. It is actively developed and uses open standards (so it's easier for programmers to work with it). This means that there are scripts and add-ons available for Firefox, and few compatibility issues with websites. Firefox is available for Windows, Mac OS X and Linux. There is also a mobile version that runs on Android.
SeaMonkey and Firefox were the first web browsers to support userscripts, thanks to the development of the Greasemonkey extension. For this reason, most userscripts can be run on Firefox, using Greasemonkey or other userscript managers, and you may often hear userscripts referred to as "Greasemonkey scripts".