Add Firefox notes for mspace@width (#17066)
Add Firefox notes for mspace@width (#17066)

Taken from https://developer.mozilla.org/en-US/docs/Web/MathML/Element/mspace#gecko-specific_notes
chrisdavidmills/browser-compat-data,mdn/browser-compat-data,mdn/browser-compat-data,chrisdavidmills/browser-compat-data