Description of problem:
I have no idea what happened, but suddenly the browser is unable to render some pages correctly. When I try to open such a page, almost no text is displayed correctly. No error dialog is displayed. The following error messages are printed to the console when I enter www.google.cz:

(firefox:8835): Pango-WARNING **: shaping failure, expect ugly output. shape-engine='BasicEngineFc', font='Liberation Sans 17.3330078125', text=' '

(firefox:8835): Pango-WARNING **: shaping failure, expect ugly output. shape-engine='BasicEngineFc', font='Liberation Sans Bold 17.3330078125', text='Vyhledávání'

(firefox:8835): Pango-WARNING **: shaping failure, expect ugly output. shape-engine='BasicEngineFc', font='Liberation Sans Bold 21.3330078125', text=' Česká republika'
...

I tried creating a new user profile, but that didn't help. I might also add that the problem originally occurred with a testing version of Firefox and the pango warnings weren't printed - instead, Firefox printed 'failed to create drawable'. When I downgraded to the latest public version in RHEL 6, the pango warnings started to occur.

Version-Release number of selected component (if applicable):
firefox-3.6.26-1.el6_2.i686

How reproducible:
Like I said, I don't know what triggered these problems. I haven't updated any rendering library, the X driver or the kernel recently. The hardware should be all right, though. Let me know if you need more information about the system.