Every now and again I get this problem where Firefox won’t render text correctly (on a Debian/stretch system). Most websites are fine, but the odd site just shows up with blanks where the text should be. Initially I thought it was NoScript, but turning that off didn’t help. Daniel Silverstone gave me a pointer today that the pages in question were using webfonts, and that provided enough information to dig deeper. The sites in question were using Cantarell, via:

src: local('Cantarell Regular'), local('Cantarell-Regular'), url(cantarell.woff2) format('woff2'), url(cantarell.woff) format('woff');

The Firefox web dev inspector didn’t show it trying to fetch the font remotely, so I removed the local() elements from the CSS. That fixed the page, letting me pinpoint the problem as a local font issue. I have fonts-cantarell installed so at first I tried to remove it, but that breaks gnome-core. So instead I did an fc-list | grep -i cant to ask fontconfig what it thought was happening. That gave:

/usr/share/fonts/opentype/cantarell/Cantarell-Regular.otf.dpkg-tmp: Cantarell:style=Regular
/usr/share/fonts/opentype/cantarell/Cantarell-Bold.otf.dpkg-tmp: Cantarell:style=Bold
/usr/share/fonts/opentype/cantarell/Cantarell-Bold.otf: Cantarell:style=Bold
/usr/share/fonts/opentype/cantarell/Cantarell-Oblique.otf: Cantarell:style=Oblique
/usr/share/fonts/opentype/cantarell/Cantarell-Regular.otf: Cantarell:style=Regular
/usr/share/fonts/opentype/cantarell/Cantarell-Bold-Oblique.otf: Cantarell:style=Bold-Oblique
/usr/share/fonts/opentype/cantarell/Cantarell-Oblique.otf.dpkg-tmp: Cantarell:style=Oblique
/usr/share/fonts/opentype/cantarell/Cantarell-BoldOblique.otf: Cantarell:style=BoldOblique

Hmmm. Those .dpkg-tmp files looked odd, and sure enough they didn’t actually exist. So I did a sudo fc-cache -f -v to force a rebuild of the font cache and restarted Firefox (it didn’t seem to work before doing so) and everything works fine now.

It seems that fc-cache must have been run at some point when dpkg had not yet completed installing an update to the fonts-cantarell package. That seems like a bug - fontconfig should probably ignore .dpkg* files, but equally I wouldn’t expect it to be run before dpkg had finished its unpacking stage fully.