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Hi, I tried all the above userChrome.css ways (Firefox 57.0b12 64-bit in Linux Mint) but couldn't affect the presence of the horizontal tabbar. Anyone tried in linux and succeeded?

Ok, this worked - thanks. I'm having the empty bar issue (described by RalfJung) both in normal and private windows but that's not much of a problem.

I would vote for the option of disabling the loading of last file on startup. Is that possible or is a question of a feature request?

Sorry but couldn't find any such option in Preferences > Application. Am I missing something? 

It seems that the same problem appears with normal Firefox (on Linux Mint with a dark gtk theme):  The Firefox theme is applied only on the...

I vote for the previous behaviour (pinned tabs not scrolled). Or at least an option for it.