While working on a recent assignment for creating a custom master page, the left side quick launch area needed to be hidden. So, I put the below statement in my masterpage's associated CSS file to override the coreV4.css default behavior:
Saved my css changes in the SharePoint Designer and checked my changes, while it looked fine on the IE 8+, on Mozilla browser, my page content was coming out / exceeding the right boundary as I was making a centered masterpage.
I used Mozilla's Toggle Tools under developer tools menu and found that the left panel was still occupying space on left and making the page content shift to the right. To fix the space occupied by the left panel, I put the below statement in the custom CSS and it did the trick on Mozilla. Do not forget to save the changes to the CSS after you have made the modifications and then refresh the page.
You can even try