I've checked it again, using the font preferences rather than the zoom
setting, and I can reproduce the problem.

Part of the problem stems from using pixels to set the margin, rather
than ems (or whatever the text box is based on).  However, although the
margin (at least visually) scales up evenly, the fonts themselves do
not.  Arguably this is a defect in Firefox, or maybe even the HTML specs
themselves.

Additionally, that only seems to control the visual margin.  I've yet to
figure out what controls the layout (such as wrapping the Go button).