Firefox (actually Iceweasel since I'm on Debian) fails to render some texts properly. It only seems to happen on some sites, for example on Stack Exchange sites everything looks pretty normal. I don't have the same issue when running Firefox on Windows.
This is what it looks like on Firefox:
For comparison, this is how Chromium renders the same text:
How can I fix this?
I'm running Iceweasel 44.0 on Debian x64.