Before you say anything, I have checked that caret browsing is off.
There is one site I use that won't now let me scroll using the up down arrows on my laptop. I have tried clearing the cache and this works, but only for a few seconds. It's been fine in the past, although there have been several Firefox updates recently. Thoughts, ideas?
The most likely explanation is there is something within the site (which could be any number of things) hijacking your keystrokes. You might be able to test for this by clicking a non-interactive space like the site background to see if it gives you a temporary fix.