There are two reasons for the text over lapping -
1) the main body text and 1st level heading in the code of each page is in almost exactly the same position as you had on the old site, although it is in a significantly different physical position in the page structure. This is to maintain the SEO used on the old site- i.e the most relevant content appears right beneath the body tag so search engines know that its important content. The affect of this was initially to cause some overlapping with the bottom menu. I've compensated for this although I still need to make a minor adjustment to the home page and then we're 100% there for normal viewing.
2) Firefox unlike Internet Explorer allows the re-sizing of text even if I specify an exact font size in either pixels or pt - there's no way of changing how the browser behaves - for example; exactly the same thing happens when you open Yahoo in Firefox and increase the font size to above the mid-level.
The way round it is to not specify the height of tables or div tags so the height of the table/div tag extends as the text size increases, but its an imperfect science. One small change to 1 class in the css file solves about 90% of the problem for us and make the pages look presentable even at very high text sizes, although still with some overlapping on the bottom menu.