The fix for this is in 1.5.dfsg+1.5.0.2-0ubuntu2. It should fix it both
with MOZ_DISABLE_PANGO=1 (which is what I've tested) and in epiphany et
al.
** Changed in: firefox (Ubuntu)
Status: Confirmed => Fix Released
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Uses nimbus for sans-serif font
https://launchpad.net/bugs/42559