I am having exactly the same issue. I am using a mac running OSX v10.9.5. I am unable to scroll through my inbox while using the office 365 online outlook service (outlook.office365.com) with either a mouse or the built in trackpad. This issue only happens on my mac, and has only started happening within the last week.
I have to manually click and drag the scroll bar to scroll, which is nearly useless since it appears to proportionally scroll through the messages, which means if I have a large number of inbox messages, moving the scroll bar even a pixel or two jumps down nearly 50+ messages.
This is the only site I'm having the issue on (that I have found).