You can't just expect a menu to reach full width because you have tweaked some padding t make it fir in one browser. Text width varies considerably between browsers by as much as 50px on a single line so you cannot just assume that if it first in one browers it will fit in another.
If you have to have a menu an exact width then you will need to class the individual menu items and give them a pixel width that matches exactly to the space available. You can center the text with text-align:center and no side padding will be needed. This means that wider text will be accommodated because it can expand without causing problem as there is some leeway in the width of each item unlike when the whole width is made of content and padding.
Alternatively for good browsers (ie8+) use display:table-cell as mentioned in my last post in this thread.