with Australis browser.css have this rules to control the way tabs are overlapping .tab-background-middle { -moz-box-flex: 1; background-clip: padding-box; border-left: 15px solid transparent; border-right: 15px solid transparent; margin: 0 -15px; } .tab-background, .tabs-newtab-button { /* overlap the tab curves */ -moz-margin-end: -15px; -moz-margin-start: -15px; } when layout.css.devPixelsPerPx is not 1 then actual border width can be different from 15px, for me on windows 7 and layout.css.devPixelsPerPx = -1.0, the actual border width is 14.4px. when is set layout.css.devPixelsPerPx to 1 the border width is 15px, with layout.css.devPixelsPerPx=1.5 the border width is 14.6667px when i apply different background color to tab-background-start, tab-background-middle and tab-background-end, there are visible vertical gaps on both tab-background-middle side