Mozilla today launched Firefox 51 for Windows, Mac, Linux, and Android.
The new version includes a new warning for websites which collect passwords but don't use HTTPS, WebGL 2 support for better 3D graphics, and FLAC (Free Lossless Audio Codec) playback.
Mozilla doesn't break out the exact numbers for Firefox, though the company does say "half a billion people around the world" use the browser.
In other words, it's a major platform that web developers target -- even in a world increasingly dominated by mobile apps.
I always use this browser myself.