Mozilla Firefox has features that allow it to be distinguished from other web browsers, such as Chrome and Internet Explorer.
Major differences
To avoid interface bloat, ship a relatively smaller core customizable to meet individual users' needs, and allow for corporate or institutional extensions to meet their varying policies, Firefox relies on a robust extension system to allow users to modify the browser according to their requirements instead of providing all features in the standard distribution.
While Opera and Google Chrome do the same, extensions for these are fewer in number as of late 2013. Internet Explorer also has an extension system but it is less widely supported than that of others. Developers supporting multiple browsers almost always support Firefox, and in many instances exclusively. As Opera has a policy of deliberately including more features in the core as they prove useful, the market for extensions is relatively unstable but also there is less need for them. The sheer number of extensions is not a good guide to the capabilities of a browser.
Protocol support and the difficulty of adding new link type protocols also vary widely across not only these browsers but across versions of these browsers. Opera has historically been most robust and consistent about supporting cutting-edge protocols such as robust file sharing eDonkey links or bitcoin transactions. These can be difficult to support in Firefox without relying on unknown small developers, which defeats the privacy purpose of these protocols. Instructions for supporting new link protocols vary widely across operating systems and Firefox versions, and are generally not implementable by end users who lack systems administration comfort and the ability to follow exact detailed instructions to type in strings.
Web technologies support
Firefox supports most basic Web standards including HTML, XML, XHTML, CSS (with extensions), JavaScript, DOM, MathML, SVG, XSLT and XPath. Firefox's standards support and growing popularity have been credited as one reason Internet Explorer 7 was to be released with improved standards support.
Since Web standards are often in contradiction with Internet Explorer's behavior, Firefox, like other browsers, has a quirks mode. This mode attempts to mimic Internet Explorer's quirks modes, which equates to using obsolete rendering standards dating back to Internet Explorer 5, or alternately newer peculiarities introduced in IE 6 or 7. However, it is not completely compatible. Because of the differing rendering, PC World notes that a minority of pages do not work in Firefox, however Internet Explorer 7's quirks mode does not either.
CNET notes that Firefox does not support ActiveX controls by default, which can also cause webpages to be missing features or to not work at all in Firefox. Mozilla made the decision to not support ActiveX due to potential security vulnerabilities, its proprietary nature and its lack of cross-platform compatibility. There are methods of using ActiveX in Firefox such as via third-party plugins but they do not work in all versions of Firefox or on all platforms.
Beginning on December 8, 2006, Firefox Nightly builds pass the Acid2 CSS standards compliance test, so all future releases of Firefox 3 would pass the test.
Firefox also implements a proprietary protocol from Google called "safebrowsing", which is not an open standard.
Cross-platform support
Mozilla Firefox runs on certain platforms that coincide OS versions in use at the time of release. In 2004 version 1 supported older operating systems such as Windows 95 and Mac OS X 10.1, by 2008 version 3 required at least OS X 10.4 and even Windows 98 support ended.
Various releases available on the primary distribution site can support the following operating systems, although not always the latest Firefox version. Various versions of Microsoft Windows, including 98, 98SE, ME, NT 4.0, 2000, XP, Server 2003, Vista, 7, 8 and 10. OS X Linux-based operating systems using X.Org Server or XFree86
Builds for Solaris (x86 and SPARC), contributed by the Sun Beijing Desktop Team, are available on the Mozilla web site.
Mozilla Firefox 1.x installation on Windows 95 requires a few additional steps.