If I comment out

update("background-color", "yellow");

The "after-styled" buttons load in their default style with a 2.4px border vs. the "inline-styled" buttons with a 1.6px border.

With the script is re-enabled, you can see that Firefox isn't adjusting the overall size and position of the recolored buttons. (Screen shot attached.)

(The odd thing is that the default style sheet has a 2px width border so why would the borders diverge so much? The divergence seems to be different at different zoom levels. Some different kind of logic is being applied that I don't understand.)

Forcing a repaint by zooming in and back out fixes the size of the buttons. I don't know whether there is a way to trigger a repaint as part of your script.

Specifying height and width CSS properties for the buttons makes them of uniform size. I still don't understand why the buttons should be of different height when all unstyled (as they are all the same height in other browsers). Width I can see varying according to content. Note that IceCat, based on a much older version of Firefox, gets it (what I think is) right, so It would seem some change made to Firefox between version 38 and version 48 affects either rendering of buttons inside table cells or tables inside table cells.