Recently I have stumbled upon a problem that cyrilic domain names would not show up in my Firefox's address bar.
After looking through various blogs and forums, reading people's complaints and comment about stupidity of this idea in the first place I have found a solution.
It seems that IE works OOTB with IDN's but Firefox does not, which is deliberately done to prevent phising scams. So, it was for a good cause originally - to bad that implementation of this "good cause", frankly sucks. I mean they could have done a pop up message or something to let you know and make a choice. In reality what we are forced to do is:
1. open "about:config" page by putting this in address box.
2. you'll see a warning message (that has same styling as other messages in Firefox, I guess everybody is a standup comedian these day, but a little humor won't hurt I guess). Proceed to show the preferences.
3. Now you need to add your top domain that is not showing to the whilelist of IDN's. There are already some built in.
4. Right click and select Add, Boolean
5. Add name of preference in the following format: network.IDN.whitelist.[top level domain], so for Ukrainian domains (*.com.ua, *.ua etc...) you add network.IDN.whitelist.ua
6. Select value true.
That's it folks!