Basically, there's another problem that seems very similar on the interaction between your editable grid and your comment tabs where selecting from any of the "Light" column's select boxes can sometimes cause the comment box to open up. This has worked almost every time in FireFox --
Open a fresh copy of the grid page
Click on any of the text in the paragraph directly above the grid.
Click a different word just to make sure.
Click on a grid cell that says "Mostly Shady"
Click on the arrow to drop down your choices
Click on "Sunny"
Watch as the comment box opens up...
I wish I could help more, but hopefully this will give you some more to go on in diagnosing the underlying issue.
FireFox appears to way off on it's click/mousedown events with certain absolute positioned elements. stopPropagation on the click event on the select may correct that particular issue. I will check it out.