Firefox 23.0.1 fieldset overflow

Firefox 23.0.1 fieldset overflow

Firefox 23.0.1 is breaking my fieldsets (or to be more accurate, I am not
writing my fieldset values correctly).
Fieldset previously worked as such.
Items with width totaling 900px or below are stored on the same row.
Fields that exceed 900px (including the field that exceeded the width)
moved to the next row starting from the left.
This worked in all browsers and life was merry.
Firefox 23.0.1 now takes the first field to exceed the 900px fieldset
width and moves it down to the next line but floats it to the right. Items
following will move down a further line and start to float from the left
What should look like this
| 100px 100px 100px 100px 100px 100px 100px 100px |
| 150px 100px 150px |
Now looks like this
| 100px 100px 100px 100px 100px 100px 100px 100px |
| 150px |
| 100px 150px |
Does anyone know where I am going wrong?