Mouse hover is more like mouse hibernation now. You have to wait until the Winter is over for it to hot track the items.
This might have to do with your machine specs and internet connection. Mouse hover is instant for me (I'm on a 13MB/sec broadband connection with a core i5 8GB RAM chromebook).
Mostly it seems to be Firefox newer than 42.0.
I see what you meant now. I have just experienced the same problem in Chrome and IE on a powerful Win7 machine. It seems like Outlook is waiting for some stuff to download before it can display the mouse hover.
I think it's a Windows (OS) problem rather than a Firefox or machine specs problem, as on a lighter OS such as Chrome OS (Chromebook) all this is instant. Maybe Outlook.com is just too resource-intensive for older OS's.
This is a bit ironic though as Outlook is an MS product and so it should have the best performance on its own OS. I don't have time to test it now but I presume maybe it's faster in Edge on Win10?