In all Windows 7 themes that I've tested (Aero, Basic, Classic), the tab bar's toolbar button :hover borders and scroll button borders (and border effects) overlap the nav bar by 1px. In non-maximized mode of Aero, the scroll buttons' border effects extend an additional pixel, so the bug becomes much more visible. This additional pixel is due to: >#main-window[sizemode=normal]:not([inFullscreen=true]) #TabsToolbar[tabsontop=true]:not(:-moz-lwtheme), >#main-window[sizemode=normal]:not([inFullscreen=true])[disablechrome] #TabsToolbar { > margin-bottom: -1px; I would consider at least the latter issue (the additional pixel) to be definitely worth fixing for Firefox 4.

this makes the appearance perfect, but those are probably the longest selectors I have ever seen :( Possible alternative could be to just set the 1px bottom border on the original -moz-border-image, is perfect in normal mode, when maximized the difference is really really faint (the line is detached 1px from the border, but doesn't look bad)

The other approach. Feel free to throw these away if they are useless :)

The real problem is that the buttons have negative vertical margins, to prevent them from increasing the tab bar height. I can probably work around it in this case, but it would be better if the margins weren't needed in the first place.

Actually, I think I'm going to just get rid of the toolbar button appearance to avoid the hack altogether...

Comment on attachment 511898 [details] [diff] [review] patch Could we use this same scaleX(-1) technique to remove the need for tab-arrow-right.png and the right half of tab-overflow-border.png in pinstripe and gnomestripe too?

Pinstripe, probably yes. Gnomestripe uses native scroll arrows which I wouldn't want to mess with.

Could we use the background-image: -moz-linear-gradient(...); background-size: 1px 100%; technique to eliminate the need for tab-overflow-border.png entirely?

Yes. Could be done in a follow-up bug.

Comment on attachment 511898 [details] [diff] [review] patch Looks good to me.

Comment on attachment 511898 [details] [diff] [review] patch This looks good! :)

Mozilla/5.0 (Windows NT 6.1; rv:2.0b12pre) Gecko/20110216 Firefox/4.0b12pre Verified issue and it's no longer present.