You can get it for free at the Mozilla website. The download link will automatically detect your operating system and language. If you’d like to download a different language or system, click the Systems & Languages link underneath the Download button. Once the download is complete, the DMG file should open automatically. If it does not, click the downloaded file on your desktop. Drag the Firefox.app file into your Applications folder. Hold down control and click the Firefox window. Select Eject “Firefox”. To add Firefox to the dock for quick access, click and drag the icon from your Applications folder onto the dock. You will be warned that the program was downloaded. Confirm that you want to open it. Firefox will ask you if you want to set it as the default browser. After making your choice, the browser will launch.

TL;DR in Chinese:
下载火狐浏览器。 打开DMG文件。 安装应用程序。 点击将火狐浏览器图标从应用程序文件夹拖拽到停靠栏上，来创建快捷打开浏览器的方式。 打开火狐浏览器。