I am getting some strange grid sizing behaviour as well, but it occurs in Firefox and IE.
Often the first column of the grid will display very large, squishing everything else out to the right. It gets fairly predictable and will happen in both Firfox and IE. I've tried to make sure ALL of my columns are set for a reasonable % width value to avoid confusion and things smooth out once there is some content in most of the cells.
Firefox is making huge changes to the browser and new updates are coming out frequently, with a major release in the works. I'm on the bleeding edge on some of my machines, so I'm not sure which version I'm running.
Having struggled through browser display issues even using very simple W3C compliant html code and very clean CSS stylesheets, I usually chock these quirks (not to be confused with quirks mode in IE) up to browser bugs. They are much better today with floats and the box model, but still can be a nightmare to debug, particular for cross browser support.
I'll see if I can characterize something.
Cheers,
James