llvmpipe has more complete EGL support than the Android software renderer, so more apps work with it, like Firefox-based browsers or more recent webviews. Unfortunately, llvmpipe is still too slow to be the default renderer, but it is possible to switch back and forth between llvmpipe and the Android software renderer.
Make sure ADB is running as root and the system partition is mounted writable.
The following command switches the software renderer implementation:
adb shell "grep -q "ro.libagl=1" /system/build.prop && sed "s/ro.libagl=1/ro.libagl=0/" -i /system/build.prop || sed "s/ro.libagl=0/ro.libagl=1/" -i /system/build.prop"
Then reboot the device. llvmpipe should now be enabled. To switch back to the default Android software rederer, run the above command again and reboot the device.