This was brought to our attention from a customer regarding future browser support:
In versions 36 and later, several users are reporting problems with Data Tables in Firefox. When editing a table field, a horizontal scroll bar always appears, covering up text being entered. I know only Firefox version 28 is supported, but wanted to report my findings for a future fix.
Using Firefox development tools, the customer narrowed down the issue to two CSS classes which cause horizontal over-sizing of the data field by 7-8 pixels in gtx-all.css.
horizontal padding in the class beginning on line 969:
padding: 1px 3px;
background: repeat-x 0 0;
border: 1px solid;
Border size in the class beginning on line 1127: