It took me half an hour to understand why this ugly thing happens. I added a new iframe tag into my existing website and it didn’t want to load the content of it. It remained blank. In other browsers (Safari, Chrome) it worked as expected.
Found many forums and bug lists about this thing, but nothing helped me. Finally I tried to open the same page from another domain and it worked.
What is the trick? Easy.
First time I tried to load the page using a link like:
and second time:
So it seems that for Firefox it is very important that you open your websites from sites with domain names.
Briefly: use a valid domain name instead of ip address and it will work