hello dcde, please see if resetting firefox can address the issue...

Resetting firefox does not help to fix the problem. Still no websites are being loaded or shown in the browser.

any new suggestions?

please also try to go through the steps at Firefox can't load websites but other browsers can

I changed to Firefox ESR 17.0.2 and everything works again. I assume there is some trouble with OS X 10.7.5 and Firefox 18.

i don't know if this is applying to your situation but there are some bugs when you're establishing a connection through proxys, which should be addressed in firefox 18.0.1 ...

That could apply to my situation since I have to use proxys. I will try out 18.0.1 when it is released. Thank you for your help.