Install this: Missing E Warning Remover.
I just logged on Tumblr this morning to find my scroll wheel was gone. I couldn’t page down or use my arrow keys either. A quick google search revealed that a bunch of people are having this issue.
It looks like it’s connected to the warning that Tumblr shows Missing E users periodically to try to scare us into uninstalling the addon that gives us features Tumblr itself doesn’t have. It *seems* like when Tumblr tries to pop that message up that it’s not getting displayed properly and poof we can’t close it so we can’t scroll down. I’m not 100% sure that’s what’s happening but this fixed it for me.
Hope this helps someone! D: