Text substitution (System prefs in macOS) is a great way of quickly adding oft repeated phrases and unicode glyphs that aren't easily accessible on the keyboard. Eg GWP₁₀₀ or m² which seeing as I'm typing this in Firefox not Safari or Chrome I have to copy and paste from the system prefs. Not sure why Mozilla in their wisedom ignore macOS software conventions and frameworks in implementing text? Is there an extension that adds this standard macOS feature back into Firefox? Is there some way to Feature Request it from the programming community who write the FF source code or extensions. See image for a few of the text substitutions I use regularly.