You can't very easily so don't even try This is css not tables and both have different characteristics. Theres not a lot of point in making a css layout behave exactly like a table because you would be better off using a table
how can I create the table functionality using divs?
As I mentioned above this is a feature of css that tables can't replicate and it is just trying to keep your elements available without scrollbars appearing. When there is no space left the float drops below. Try resizing this sitepoint page and you will see the menu at the top break into pieces and line up under each other.
If you don't want the page to get any smaller then the only fix is to use min-width for mozilla and some js for ie (or expressions (which are proprietary ie code) .
Personally I would just put your right hand header image in the background of the header and then it will just slide underneath the left logo when it gets too small. Alternatively tou could absolutely place the right image to the right and then set the z-index so that it slides under the left at small sizes. I would use the background version because it cleans up the html.
Floats are basically removed from the flow (albeit in a special way) but that means that they will float out of their container because the container doesn't know they are there.
2) For some reason the background colour doesnt seem to render in Firefox
This is by design and the answer is that you have to clear the floats before the closing div of the parent container. This will force the background to encompass the floats.
The easiest way to do this is with a class as follows.
The same goes for your footer.
<div id="mainHeader"><div id="logoImageHolderLeft"><a href='http://24601.net/'><img src='images/mainlogo_left.jpg' alt='Left Logo' border=0></a></div><div id="logoImageHolderRight"><a href='http://24601.net/'><img src='images/mainlogo_right.jpg' alt='Right Logo' border=0></a></div> <div class="clearer"></div>
</div><!--closing div of parent container -->
Hope that explains it all satisfactorily