Unsure why, but the arrow examples you have there show up as unicode code blocks for me in Firefox on a Mac. Probably one of the reasons not to use them like that!
The more commonly-used characters are ▲ and ▼, BLACK {UP/DOWN}-POINTING TRIANGLE (U+25B2, U+25BC).
Parent comment used much newer characters 🞁 and 🞃, BLACK {UP/DOWN}-POINTING ISOSCELES RIGHT TRIANGLE (U+1F781, U+1F783). These were added in Unicode 7, June 2014, where the others have been around for thirty years. I’m mildly surprised at lack of font support on macOS. For me they’re being rendered with Noto Sans Math, and completely wrong: they’re not being drawn as right-angled triangles at all (the angle is about 53° rather than 90°)! I’m guessing the commenter also had a bad font on them, because as a right angle it’d look all wrong for this purpose. (I’ve filed a bug report at https://github.com/notofonts/math/issues/47. Curiously, Noto Sans Symbols 2 also includes the character and gets its shape right, but Noto Sans Math is higher in the font fallback list (`fc-match sans-serif --all`: 881 lines, NotoSansMath-Regular.ttf is 52nd and NotoSansSymbols2-Regular.ttf is 89th.)
There are also ⯅ and ⯆, BLACK {UP/DOWN}-POINTING TRIANGLE CENTRED (U+2BC5, U+2BC6). And more, but of roughly-these-sized ones, these are the ones.
They even work on the dinosaur age Firefox in Sailfish OS. I guess it's just whether the fonts used have the character or not, nothing to do with Firefox as such.
