Start Firefox in Safe Mode {web Link} by holding down the <Shift>
(Mac=Options) key, and then starting Firefox. A small dialog should appear. Click Start In Safe Mode (not Refresh).

Is the problem still there?

Still there. The safe mode dialog also has from the issue.

You can do a font test to see if you can identify corrupted font(s).

• http://browserspy.dk/fonts-flash.php?detail=1

The fonts Padauk and Padauk Book seem to have the same problem. But changing the font in Preferences doesn't do anything.

Does it has effect if you temporarily remove these two fonts from its installation folder?

Removing the fonts fixes the problem! But if they are present, then the problem comes back, regardless of my font settings. Is the font used by the menus/tabs/dialogs specified somewhere else?

Did you check what default fonts are set in the Linux font settings?

• gnome-appearance-properties

You can try to set gfx.font_rendering.fontconfig.fontlist.enabled = false on the about:config page.

Thanks very much! The problem was specific to Padauk, which happened to be my system's default sans serif font.