Firefox vs Safari
Firefox and Safari are cross-platform internet browsers.
Apple claims that Safari is currently the fastest browser available and is confirmed on both Windows and Mac operating systems . The performance difference is especially noticeable on SSL-encrypted websites, where Firefox suffers from sluggish performance.
Firefox and Safari have many features in common, but there are some features that exist in one browser and not the other.
 Firefox lets you:
- install extensions that provide additional features and functionality from a huge selection of "Add-ons"
- the same that safari lets you do (using those addons)
 Safari lets you:
- perform "Full History Search", meaning that users can search not only the url but also the content of pages previously viewed
- flip through full-page previews of the sites visited in the past ("Cover Flow")
- browse and rank up to 24 favorite sites by presenting them on a single page ("Top Sites")
- "Type a word into the new Find banner below the Bookmarks Bar, and Safari shows you the number of matches and brightly highlights matching terms while dimming the rest of the page" 
- install plug-ins (such as ClickToFlash) that provide additional features and functionality, though the selection is relatively limited
Both browsers support extensions. Firefox's extension library is much larger than that of any other browser.
 Standards Compliance and Support
Standards compliance and full standards support are two distinct concepts that are often confused with one another. Standards support implies a particular set of standards have been implemented by the browser, while standards compliance implies that any part of the standard that is implemented conforms to the specifications for that functionality in the standard.
Both Firefox and Safari are substantially, if not entirely, standards compliant. Neither, however, entirely supports all relevant web standards.
Safari passes the ACID 3 test. Firefox currently passes less than 94 of the 100 test cases. This would seem to indicate greater standards support by Safari than Firefox, though the ACID tests only test for a largely arbitrary subset of complete standards.
 User Interface
Firefox 3 will support native widgets to provide a seamless user interface for every operating system. However, Firefox does not change its color to indicate whether it is active or inactive as other applications on Mac OSX do. Safari integrates well into the Mac OSX operating system.
Safari has an annoying bookmark management interface, where the bookmark manager will replace the current page of the browser instead of opening a dialog like Firefox does.
 HTML Rendering
Firefox uses the Gecko rendering engine. Safari uses the Webkit rendering engine (which was originally a modified version of KHTML).
Firefox can be run on most major operating systems including Linux, Solaris, and the various BSD Unix OSes, in addition to MS Windows and MacOS X. Safari only works on OS X. (Versions that ran on Windows were released, but discontinued due to lack of interest.)
Firefox is a free/open source application, distributable under the terms of three licenses: the MPL, the GNU LGPL, and the GNU GPL. Apple has released Safari's rendering engine, WebKit, as a free library (under GNU LGPL and BSD license), but the application itself is not free/open source software.