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.
If you right-click on the Firefox toolbar, select “Customize Toolbar…” and then in the bottom left corner tick the “Title Bar” checkbox, does that change anything about it?