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

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.