Why is Firefox missing the border on some HTML tables

Question

I am using Firefox 3.5.7 and I have the same CSS used in multiple HTML tables, but there are some examples where parts of the borders are not shown.

What makes no sens

Answer 1

This was caused by the table (with a missing bottom-border) to be inside a div...The border apparently didn't get calculated in the table height, so the border was there but wasn't shown.

Giving the surrounding div a margin-bottom from 1px solved the problem.