Update : there is a companion extension that supports scrolling among the tabs using the wheel of the mouse: https://addons.mozilla.org/en-US/firefox/addon/tree-style-tab-mouse-wheel/
Therefore the Autohotkey solution is now obsolete.
------------------------- old review -----------------------------
One more 5 Stars and a bigup for Piro!

I really like to be able to scroll the tabs in my Firefox. Since v57, it's not possible anymore, so here is a solution found on the"Tab Wheel Scroll" extension page :

1) Get Autohotkey
2) Create the following script:


#IfWinActive ahk_class MozillaWindowClass
;MouseGetPos, [OutputVarX, OutputVarY, OutputVarWin, OutputVarControl, 1|2|3]
~WheelDown::
MouseGetPos X, Y
if ( X < 205 )
SendInput ^{PgDn}
Return
~WheelUp::
MouseGetPos X, Y
if ( X < 205 )
SendInput ^{PgUp}
Return

And run the script whenever you launch Firefox.
You must adjust the X value ("205" pixels in my case) to the width of your vertical tab.