Proxy problem with 2.0.0.3

I am behind a proxy that requires authentication, and under 2.0.0.1 all was working fine. After upgrading to 2.0.0.3 when I start portable firefox I get a proxy authentication dialog that is half empty - the OK and Cancel buttons are there but with no text in them, and clicking on them does nothing. Sometimes the dialog is missing all of the text entirely. Pressing Enter or Esc do nothing - all I can do is close the dialog, at which point it just reopens (again and again, I have closed it over 30 times and it still reopens).

Running a fresh install of 2.0.0.3 just hangs - the app shows up in the task bar with the firefox icon, but no text (ie. it would usually say Mozilla Firefox).