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 is a bit tangential, but for those searching for missing borders in Firefox…

I had a border-bottom of a table-row () that was missing where a background had been set on a specific cell. It turned out this was caused by a rogue position: relative on the cell, removing that (or changing it to position: inherit) fixed it.

Firefox 70.0.1