firefox-esr (68) in Sid is built upon freetype2 v2.9: https://salsa.debian.org/mozilla-team/firefox/tree/esr68/master/modules/freetype2
but firefox (69) in Sid is built upon freetype v2.10: https://salsa.debian.org/mozilla-team/firefox/tree/release/master/modules/freetype2 To avoid a frankendebian, I would rather install the above mentioned Mozilla official snap package (firefox 69), though. Good luck :-)
