Several times I noticed that I can not perform the following actions on Habrahabr:
Login ("Action not defined")
post topic ("Unidentified Error")
vote for topic
leave a comment (“Not enough comment”)
and generally perform any action related to ajax requests
As it turned out, my fault is Outpost Security Suite Pro. After unloading the latter, everything works fine. I did not understand the essence of the problem, but I did not notice any prohibited connections in the list of network activities. My FireFox is in the list of trusted applications.
I would be glad if anyone this information seems useful.