During its beta phase, it proved to be popular with its testers and was praised for its speed, security, and add-ons compared to Microsoft's then-dominant Internet Explorer 6.

[20] In November 2017, Firefox began incorporating new technology under the code name "Quantum" to promote parallelism and a more intuitive user interface.

[40] Firefox attracted attention as an alternative to Internet Explorer, which had come under fire for its alleged poor program design and insecurity—detractors cite IE's lack of support for certain Web standards, use of the potentially dangerous ActiveX component, and vulnerability to spyware and malware installation.

[citation needed] Microsoft responded by releasing Windows XP Service Pack 2, which added several important security features to Internet Explorer 6.

[43] Major redesigns of its graphical user interface occurred on versions 4.0 (Strata) in March 2011, 29.0 (Australis) in April 2014, 57.0 (Photon) in November 2017, and 89.0 (Proton) in June 2021.

Despite its improvements, these changes required existing add-ons for Firefox to be made incompatible with newer versions, in favor of a new extension system that is designed to be similar to Chrome and other recent browsers.

[47][48] On May 3, 2019, the expiry of an intermediate signing certificate on Mozilla servers caused Firefox to automatically disable and lock all browser extensions (add-ons).

[53] While users were initially allowed to revert to the old design through about:config, the corresponding key-value pairs reportedly stopped working in later builds, resulting in criticism.

[71] Firefox provides an environment for web developers in which they can use built-in tools, such as the Error Console or the DOM Inspector, and extensions, such as Firebug and more recently there has been an integration feature with Pocket.

Add-ons are primarily coded using an HTML, CSS, JavaScript, with API known as WebExtensions, which is designed to be compatible with Google Chrome and Microsoft Edge extension systems.

[77] Firefox previously supported add-ons using the XUL and XPCOM APIs, which allowed them to directly access and manipulate much of the browser's internal functionality.

[96][97] Firefox also implements "Safe Browsing,"[98] a proprietary protocol[99] from Google used to exchange data related with phishing and malware protection.

[113] However, the rewrite caused issues with their legacy extension API, which was not designed to work cross-process and required shim code to function correctly.

[114] In 2012, Mozilla launched a new project called Servo to write a completely new and experimental browser engine utilizing memory safe techniques written in Rust.

[116] The project completely overhauled Firefox's page rendering code resulting in performance and stability gains while also improving the security of existing components.

[124][125] Version 23, released in August 2013, followed the lead of its competitors by blocking iframe, stylesheet, and script resources served from non-HTTPS servers embedded on HTTPS pages by default.

[136] Also known as state partitioning or "total cookie protection", SmartBlock works via a feature in the browser that isolates data from each site visited by the user to ensure that cross-site scripting is very difficult if not impossible.

[183] In the release calendar page, a note states that Mozilla will re-evaluate the situation in early 2025 to see if another extension will be needed or not and statute about 115 ESR end-of-life then.

[233] Firefox has also been ported to FreeBSD,[234] NetBSD,[235] OpenBSD,[236] OpenIndiana,[237] OS/2,[238] ArcaOS,[239] SkyOS, RISC OS[240] and BeOS/Haiku,[241][242][243][244] and an unofficial rebranded version called Timberwolf has been available for AmigaOS 4.

In 2017, Landry began hosting packages of newer Firefox versions for OpenBSD releases from 6.0 onwards, making them available to installations without the ports system.

In March 2011, Mozilla presented plans to switch to the rapid release model, a faster 16-week development cycle, similar to Google Chrome.

Ars Technica noted that this new cycle entailed "significant technical and operational challenges" for Mozilla (notably preserving third-party add-on compatibility), but that it would help accelerate Firefox's adoption of new web standards, feature, and performance improvements.

As a result, several publicly released applications have been built from it, including Firefox's predecessor Netscape,[265] the customizable Pale Moon, and the privacy focused Tor Browser.

[278] Early Firebird and Phoenix releases of Firefox were considered to have reasonable visual designs but fell short when compared to many other professional software packages.

In October 2003, professional interface designer Steven Garrity authored an article covering everything he considered to be wrong with Mozilla's visual identity.

The release of their manifesto stated that "the Mozilla project is a global community of people who believe that openness, innovation and opportunity are key to the continued health of the Internet.

"[296] In celebration of the third anniversary of the founding of the Mozilla Foundation, the "World Firefox Day" campaign was established on July 15, 2006,[297][298] and ran until September 15, 2006.

[303] To promote the launch of Firefox Quantum in November 2017, Mozilla partnered with Reggie Watts to produce a series of TV ads and social media content.

Lifehacker timed how long browsers took to start and reach a page (both right after boot-up and after running at least once already), timed how long browsers took to load nine tabs at once, tested JavaScript speeds using Mozilla's Dromaeo online suite (which implements Apple's SunSpider and Google's V8 tests) and measured memory usage using Windows 7's process manager.

Firefox also scored the highest on the "non-performance" index, which measured memory efficiency, reliability, security, and standards conformance, finishing ahead of Chrome, the runner-up.