If Firefox 68 is too slow, consider these "lightweight" alternatives that were popular during the Android 4.2 era:

Firefox APK Android 4.2.2 (Jelly Bean) is a common goal for those reviving older tablets or phones. Because modern versions of Firefox require much newer software, you must use a specific legacy version to maintain compatibility. Recommended Version: Firefox 55 For Android 4.2.2, Firefox 55.0.2

If your device is (rare, e.g. Asus ZenFone 2), choose the x86 variant.