From the MathML specification, in the description of the mathvariant attribute:
"Certain combinations of character data and mathvariant values are equivalent to assigned Unicode code points that encode mathematical alphanumeric symbols. These Unicode code points are the ones in the Arabic Mathematical Alphabetic Symbols block U+1EE00 to U+1EEFF, Mathematical Alphanumeric Symbols block U+1D400 to U+1D7FF, listed in the Unicode standard, and the ones in the Letterlike Symbols range U+2100 to U+214F that represent "holes" in the alphabets in the SMP, listed in Section 7.5 Mathematical Alphanumeric Symbols."
STIX is supposed to cover any glyphs used in scientific texts and thus the Arabic Mathematical Alphabetic Symbols should be covered too. See http://www.unicode.org/charts/PDF/U1EE00.pdf I'm guessing the corresponding "normal" mathvariant in 0600-06FF should be covered too.
Gecko bug: https://bugzilla.mozilla.org/show_bug.cgi?id=114365.
Khaled Hosny has designed some glyphs for Amiri and XITS that could be used: https://github.com/khaledhosny/xits-math/issues/36