[html, css, javascript] td width acting like its not there!

[Lea Verou]

I have a table with some other tables inside it's cells.
I wanted the inside tables to be 100% the width of the table cells, and of course I want the table cells that contain them to be 100% of their table width (minus the padding)!
However, I've noticed a very weird issue, that only occurs with firefox (and perhaps Opera but not IE7). The table cells of the outer table are less wide than the table (and I'm not counting the padding) and consequently they make the inner table less wide than I want it to be!!
The table cells adjust to the automatic (defined by the content) width of the inner tables. No matter how much width="100%" I've added to the tds involved, they seem to ignore it completely! If the content is enough I get the desired result but if the content is too little they adjust to it regardless of any width definition I'm trying to apply.
And if wasn't strange enough, the problem occurs only after I hide and re-show the table via javascript. Via my javascript I change the css display attribute of the table to none in order to hide it and then to block in order to show it. If I change the javasript to change the display property to 'table' and not 'block' in order to show the table it shows up properly (the cells are 100% wide) but I have problems with IE so I discarded this solution. I need to work this out via display:block for the table.
Any ideas of what I could try? This issue is driving me mad