But it is "particular" to WebKit (on iOS and OSX, due to CoreText), which Firefox has nothing to do with.
Read the first comment; since when was curl a WebKit tool?
> This isn't a bug inside WebKit. It's a bug inside Apples CoreText font rendering framework.
We do use Core Text in Firefox on OS X, although not for all parts of text layout/rendering nowadays, AIUI. We're using Harfbuzz for some text shaping, so it's possible we're not using the particular Core Text APIs that hit this bug.