[Repost]The FirefoxCSS archive really should be maintained; it is a unique source of info even though many codes are partially outdated.
However, for a long time, the discussions on that sub have been dominated by spammers requesting support for extensions, or showing off useless personal setups.
Then there are the many posters who will not engage in a discussion on css code, simply expecting a quick fix regardless of the complexity of the issue; not to mention the many efforts by contributers which go unanswered.
Since hardly anyone seems to be unduly bothered by the possible demise of that sub - including our very own MrOtherGuy ;) - I assume that there is little if any interest in discussing the finer points of Firefox specific CSS coding.
It might interfere with the css style, which is creating its own navbar shadow, but that might be (partially?) overriden by current Fx versions anyways.
The issue has been discussed in a couple of other forums, seems like the code for that shadow has changed recently.
Again, maybe give this a shot, which has worked for some other users; to be safe put it at the bottom of your userChrome.css, shouldn't matter though:
If it's the box-shadow causing the issue, in my testing it will only be visible in (most?) 3rd party themes, I couldn't see it in the official Fx and system(Mac) themes either.
On the other hand, that shadow should then go all across the bar, not just show under the selected tab.
Unless the chosen theme or his custom css moved things around a bit.
Maybe check if there is a that shadow listed in the browser tools, and if so how it is applied in FX123; also if the issue occurs with every theme, or just the ones that are not native Fx or system themes.
Thanks very much for the reply; that's pretty much what I experienced; the additional tab corners basically would need to be inside the "physical" boundaries of the tab itself.
Which I found hard to do for some reason, but I'll give it another shot.
I've cobbled together some sort of preliminary "solution" - but the proper term for that thing would probably be hacky. ;)