In Firefox the total width of the inner block: width+padding = 170+10+10=190px
In Firefox the total width of the outer block: width+padding = 400+20+20=440px
In Firefox the distance between the left hand side border of both blocks: 30+20=50px
In Internet Explorer the total width of the inner block: width = 170px
In Internet Explorer the total width of the outer block: width = 400px
In Internet Explorer the distance between the left hand side border of both blocks: 30+20=50px (*).
(*)
You need to use css "display: inline;" to avoid "Internet Explorer doubled float-margin bug".
If you do not use "display: inline;" the distance will be 2x30+20=80px;