SeaMonkey
SeaMonkey is the original free, open source all-in-one web browser/email/chat suite 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 SeaMonkey, and few compatibility issues with websites. SeaMonkey is available for Windows, Mac OS X and Linux.
SeaMonkey and Firefox were the first web browsers to support userscripts, thanks to the development of the Greasemonkey extension and continued with Greasemonkey Port extension. For this reason, most userscripts can be run on SeaMonkey, using Greasemonkey Port or other available userscript managers. You may often hear userscripts referred to as "Greasemonkey scripts".