I stand by Windows Defender, and continue to recommending it.
Curiously though, since Aura asked this of you specifically, on
our own Windows 8.1 system, Defender has been deactivated because we are now trying Avast!
So far, so good!!
goro, if you
truly love Firefox's user interface, but find it
still behaves slowly on your lap top just as it does on ours, you might like Cyberfox x64!
I browse with their portable version extracted onto a USB flash drive to keep our system lighter. The choice is yours. Just be sure you choose the correct version for your CPU which you can read all about it here:
https://8pecxstudios.com/