Are you sure this is because of the columns sizes? Your CellTable seems to be within another <table> with no specified width. A quick test in pure HTML shows that Firefox extends the outer table to width:100% while Chrome doesn't (and the inner table then has the minimum size required to show the absolutly-sized columns and the table/cell borders).
Here, if your table is 100px wide, 50%=50px and 40%=40px, so the remaining 10% are the 10px taken by the first column. But otherwise (i.e. most of the time), 10% != 10px, so there would be some space left, or not even enough space to give to each column (table-width=90px -> 50%=45px and 40%=36px; 10+45+36=91px, wider than the table!). According to CSS, in the first case, the extra space will be distributed (which means your 10px-wide column might now be 15px), and in the second case, the table is wider than originally asked (in my example, it would be 91px wide rather than demanded 90px).