It's a strange error and actually it could be a Firefox issue. We have to investigate it more, because we are not able to replicate it. I have performed some google searches and it seems there are many people who get similar error for no obvious reason.
You could try to temporary disable your add-ons and see what happens. Even if the browser is not updated often, some of the add-ons are.
We are sorry for not being able to help you much. We will look forward to your reply, to see whether disabling the add-ons helps.
the Telerik team