I can confirm, that when using Firefox 3.0.1 you CAN NOT use arrow keys to select items from OpenGoo's "suggest box". Absolutely nothing happens when i hit "down" key.
Could you specify the screen where this happens?
1. Create a new task.
2. Go to task details page
3. From "Actions" menu choose "Edit"
4. Expand "Tags".
5. Start typing in a textbox entitled "Tags".
Although suggestions does show below the box itself. Unfortunately i am unable to select any of these, neither by using cursor keys nor with a mouse. Up and Down arrow keys cause cursor to move inside the textbox itself. Up moves it to the beginning of a text, Down to it's end.