Firefox is a free, open-source web browser developed by Mozilla, a non-profit organisation. People use it to visit websites, just like Chrome or Safari, but it stands out for strong privacy defaults and a long history of sticking closely to open web standards. Because it is open source, anyone can look inside and see exactly how it handles your data.
Here is why it matters to anyone with a site. Firefox renders pages using its own engine, Gecko, while Chrome and Edge share a different one called Blink. So the same code can look slightly different across them, much like the same recipe turning out differently in two ovens. That is exactly why browser compatibility testing matters: a button that lines up perfectly in Google Chrome might shift a few pixels in Firefox if the code is not solid.
Firefox also ships with privacy tools that block many trackers out of the box, and Mozilla funds itself differently from the big advertising-driven companies. For a meaningful share of users, especially privacy-conscious ones, it is the daily browser of choice.
Where Gecko earns its keep is the edge cases. A new CSS feature, a custom scrollbar, a date picker, or a video format can arrive in Blink first and reach Firefox months later, or behave in its own way. Developers who only check Chrome miss these gaps until a user reports a broken page. Firefox also has the best built-in developer tools for inspecting CSS grid and flexbox layouts, which is why many people keep it open while building even if they ship to Chrome.
At TopDevs we test every build in Firefox alongside Chrome and Safari, so a client’s site looks right and works the same for the people who do not follow the crowd.
