Our product is facing issues in Firefox, where the first label header from the left is displayed differently from the others. This does not happen in Chrome or Safari and I'm wondering if there's a fix for this. We are using the version 7.1.8.
Quickly tried to replicate the same at my end ( using out of the box rules involved in UI gallery ) and found that it is working fine without any issues.
As a next step I would suggest you to do some initial debugging using Firefox style editor and identify whether there is any style element which is adding a kind of pad or something in the column header !