參考網址：https://linuxhint.com/install_firefox_raspberry_pi/
If you want to set Firefox as the default browser on Raspbian, run the following command:
$ sudo update-alternatives –config x-www-browser
Firefox is in the list and the selection number of Firefox is 4 as you can see in the marked section of the screenshot below. Yours may be different depending on what browsers you have installed. Now, type in the selection number (in my case 4) and press <Enter>.