On of my tables is defined to be 100% wide. It also defines the widths in pixels of all but one column - my hope being that the browser will figure out the width of the undefined column, and render it properly.
IE does this and it looks great.
However, Firefox just assumes that the undefined column has a width of 0, and spreads the other columns out to make up the 100% - even though they have widths defined in pixels and FF has to break these to spread them out.
You could try a width of 100% for the remaining column.
I use a table in a navigation bar that extends the whole width without any problem. HOWEVER, I never set set widths in pixels. Try setting all the other column widths to 1%. If you need to avoid word wrapping then there are several ways to do this.