Ukraine 20% (36%), Slovakia 6% (13%), Poland 7% (12%), Czech Republic 5% (10%), Russia 14% (32%), Belarus 32% (45%), Kazachstan 32% (16%)
Also, Mobile browsers are not insignificant market: (whole) Europe 24% (50%)
Numbers from statcounter.com as of January 2013 (January 2010). I picked 2010 as it was about the time Chrome started it's push around here (I'm in Czech Republic at the moment) - ads in radio, tv, outdoors, on homepages of google.com, youtube.com; Opera was often blocked from accessing gmail, apps,; was deprived of features (by user-agent sniffing) google-wide. And FF was well established at the time, so it didn't carve much to the Opera's share.