Firefox 18.0.2 behaviour with description field in work item defect

Using RTC 4.0.1 web client with Firefox 18.0.2 I get the following problem in the Description panel.
After I have pasted text from another panel into the Description field, it is no more possible to type additional text:
As soon I enter a character the cursor jumps back to the first position of that line.
Can anybody tell me if it is a setting problem ?
After I have pasted text from another panel into the Description field, it is no more possible to type additional text:
As soon I enter a character the cursor jumps back to the first position of that line.
Can anybody tell me if it is a setting problem ?