How can I solve the problem with copying text from Google Documents?

Ctrl+C and Ctrl+X are not always working in Google Document opened with Mozilla Firefox 30. This problem first occurred 2 days ago. No changes to the software were made. The problem is not stable and permanent. I can successfully copy several lines from document and paste them somewhere else. And suddenly the next one is not copied using ctrl+c. Even ctrl+x is not working until I change the selection of the lines. Sometimes it begin to work after changing the selection, sometimes not. I'm using Google Documents for a long time. I always use Firefox to work with them. And I've never had this problem before.