I have some tables which render fine in IE and Chrome. But in Firefox some of the border arbitrarily don\'t show or have different widths. You can see an example Here. Below
I had an issue where borders were appearing when they shouldn't have been. I resolved it with this:
border-collapse:separate;
I know it's not the direct answer, but the search brought me here.
I'm posting this as the answer as I'm convinced at this point you must be just a little zoomed out in Firefox. Try pressing Ctrl+0
If you compare the two tables in the image you provided, you can see that the bottom one is slightly larger than the top one.