Oh, I did not think about that. On Android 8, you make firefox the default ... (renderer ?) for other apps but idk if it's the case for older versions of Android.
On the other hand, as others have said, these devices are generally quite painful to browse on so accessing the wweb version of many apps could be a solution, plus firefox will let you place shortcuts on your home screen . I also wonder how LineageOS works on those old devices. Could be another solution.