Well I’ve looked it up for you, apparently the W3C CSSWG standard has decided that text-transform should not apply to plain text copy and paste.
So it seems Chrome is the exception and not the expected behaviour as such.
In any case it’s not a Mendix thing, but just how different browsers have decided to implement things.
As far as I can tell this is a deliberate choice by the Firefox and IE developers – they believe the underlying text is the “correct” text that should be copied. You’ll probably have to transform the text back-end or in Javascript so you can display it without using text-transform if you want Firefox to copy it as uppercase. Related Firefox bug thread, closed as “won’t fix” https://bugzilla.mozilla.org/show_bug.cgi?id=35148