History of the browser user-agent, and why “Mozilla”

Today, I learned a fascinating history lesson—Why do all browsers’ user agents start with “Mozilla/”?

Aaron Andersen wrote the best answer in his article, History of the browser user-agent string. (archive)

Read the article above for details, but here is the brief.

The story behind why all browsers’ user agents start with Mozilla is a fascinating mix of early web browser rivalry, technical quirks, and clever tricks to ensure websites worked properly.

It began with NCSA Mosaic, the first popular web browser, which identified itself as NCSA_Mosaic/2.0 (Windows 3.1). This was followed by Netscape Navigator, which adopted the user-agent string Mozilla/1.0 (Win3.1) to signify its advanced features. In response, Microsoft Internet Explorer mimicked Netscape’s user-agent string to gain compatibility with websites that served content based on the Mozilla identifier.

As browsers evolved, they continued to include Mozilla in their user-agent strings, even if they were not based on the original Netscape codebase. For instance, Google Chrome, which uses the WebKit rendering engine, adopted a user-agent string that included Mozilla to ensure compatibility with websites expecting that identifier. This practice led to a situation where all major browsers’ user-agent strings started with Mozilla, despite their differing underlying technologies.

Over time, this redundancy became a source of confusion and inefficiency. In response, Google announced in 2020 that it would freeze parts of Chrome’s user-agent string to reduce its role in browser fingerprinting and to encourage the adoption of the more privacy-conscious Client Hints API. Similarly, Firefox began freezing portions of its user-agent string in 2023 to align with this privacy initiative. Despite these changes, the legacy of the Mozilla prefix persists in modern browsers’ user-agent strings.

Well, the user agent string is a lie. Today’s user agent strings are often a tangled mess of legacy tokens just to trick websites into working well.

“Mozilla” originally meant the giant green lizard mascot of Netscape, but now it’s just a token in the user agent — a relic of early web browser competition.