First of all, open the menu in the latest version of Android Firefox, by default it is located in the bottom right corner, since switching to Arabic will change the interface to a mirror interface, so it should now be in the bottom left corner, now click on the gear icon (اﻻص ادات) in the bottom row of the very bottom. Then select (لغة), which is the language selection screen, which is the 9th element that can be interacted with from top to bottom (if excluding the case of return). Usually the first option is the language you use every day, and even if you don't find it, you can search for your language with the magnifying glass in the upper left corner or look for your language from the list.