Kaspersky used to be the best of the 'paid for' anti-virus programs but recently it's been reported that it's now slowing many computers down quite a bit.
Try turning Kaspersky off and seeing if there's any improvement when you use Firefox to view AB. (Don't worry. It's perfectly safe to turn your anti-virus program off for a short while as long as you only visit safe sites). Go to Kasperky's settings panel, click 'Protection' (marked '1' in the image) and turn off the switch at the top right (marked '2'):
https://support.kaspersky.co.uk/images/kts_11471_01124-259192.png
(NB: Remember to turn it back on after you've tried AB with Firefox!)
If turning off Kaspersky improves things you'll need to consider using a different anti-virus program (such as Avast). If it doesn't, it might be worth trying to reset Firefox to its default settings. (Instructions available on request).