By default, Firefox for Android loads the same language that's currently set on your device. If you prefer to display another language, you can switch by following these steps:

Tap the menu button (either below the screen on some devices or at the top-right corner of the browser) . Tap Settings (you may need to tap More first) , then Language. Tap the menu under Browser Language. This is set to System Default the first time you access this setting.

Select your preferred language from the list.

Firefox for Android will switch to the language you've selected.

Sidharth Bhandari (Rubin) said

By default, Firefox for Android loads the same language that's currently set on your device. If you prefer to display another language, you can switch by following these steps: Tap the menu button (either below the screen on some devices or at the top-right corner of the browser) . Tap Settings (you may need to tap More first) , then Language. Tap the menu under Browser Language. This is set to System Default the first time you access this setting. Select your preferred language from the list. Firefox for Android will switch to the language you've selected.

I know how to change the language. The problem is that I cannot find Bengali (Bangladesh) language in the language list.