Mouse offset in Firefox since update

Since the update, if I snap Firefox to the top the mouse seems to be offset a number of pixels compared to where the actual cursor is. It also affects the content in sites, not just stuff in Firefox.

Edit: Looks like it's not just when it's snapped, but as soon as it passes about the top 90% of the screen. Just took a screenshot, where you can see the cursor is hovering over Other Bookmarks, but it's activating the menu above it.