The newly-activated OOjs checkboxes "This is a minor edit" and "Watch this page" are off-center whenever I load any page on any Wikipedia.
However, if I zoom in and out, they fix themselves. See it in action:
I'm using Firefox 54.0.1 on OS X El Capitan (10.11.6).
Edit: It also happens with other OOJs checkboxes, e.g. in the Beta features tab of Special:Preferences.
Edit 2: Here's another example: The Visual Editor save dialog box. It sometimes takes a few tries to get the checkboxes aligned.