I’m using a Windows 8 tablet (the Linx 10 if you’re curious) and found that the close buttons in Firefox were a bit small and difficult to press.

I’ve fixed this by adding the following to my userChrome.css file:

.tab-close-button
{
 border-radius: 20%;
 background: rgba(0,0,0,0.1) !important;
 width: 32px !important;
 height: 32px !important;
}