pip install -U selenium
I had this same issue with Firefox 34.0.5 (Dec 1, 2014) and upgrading Selenium from 2.42.1 to 2.44.0 resolved my issue.
However, I've have since seen this issue again, I think with 2.44.0, and another upgrade fixed it. So I'm wondering if it might be fixed by simply uninstalling and then re-installing. If so, I'm not sure what that would indicate the underlying problem is.