I tried adding comments on your site in both FF and IE8.
I see the problem, but am not sure of the solution. The URL to the posts have a bunch of percent encoded values because of the Russian characters. When adding a comment, an asynchronous POST to the server is done. The POST is sent to the
correct URL in FF. But in IE, a 404 error is occurring because it's trying to do a POST to:
in Firefox, the POST is being done to:
and there is no 404. This URL matches the URL in the address bar.
As a workaround, I would try using non-Russian SLUGs when creating your posts. The post Title can still be Russian. But maybe use a non-Russian Slug. You can at least try that and see if it then works in IE8, which I'm guessing it will.