There are currently a number of strange issues using the visual editor in Firefox with regards to arrow keys. The known issues so far (this list may grow if any more are discovered):
Pressing the 'up' key anywhere in the body of a node with text will not move the cursor up by one line, it will move the cursor to the beginning of the closest node containing text above the current node.
Pressing the 'down' key inside a Code node does not move the cursor down to the closest node containing text below the current node.
Building on #2088.
There are currently a number of strange issues using the visual editor in Firefox with regards to arrow keys. The known issues so far (this list may grow if any more are discovered):