Weird formatting issue on Pro Sites checkout page affecting only Firefox

Hi, please see my attached screenshots. The one where the formatting looks okay is how the pro sites checkout grid appears in chrome, safari, and just about every other browser I've tried.

The other one, with the pro sites checkout grid hanging in barely from the left, and where the options to select payment don't even appear, is a screenshot of how the grid appears in Firefox.

Is this just my theme? Or is this a weird Pro Sites quirk?

Or is there some other issue here. Haven't noticed this problem until recently, but a lot has happened so I'm not sure if this is the result of a plugin update or a theme update or what.

WHAT I'VE TRIED: I can fix the firefox problem by selecting #psts_checkout_grid and then positioning the grid to the left -400 pixels or so, but then of course it messes with the format in chrome, safari, etc. Please help. Thanks.