Firefox updated itself to 57 and made tabs impossible to see again.
They broke that quite a while back, but before 57 you could use "classic theme restorer" to make them visible again. But 57 stopped it from working and there is apparently no fix.
So had to switch back to 56.
And then they also brag about a lie on their website "Set up Firefox your way. " when you cannot even set tab borders anymore.
Click hamburger menu on the right > Add-ons >Themes>Light. It is not intuitive though. One of my colleagues updated and his browser is blue as his Windows theme is blue. Mine is black.
I like the new version as it is much faster. I use RSS and usually in the morning I click open all in tabs (usually 50 tabs) and the browser gets very slow. I notice that Slashdot pages load a bit faster now. But let's hope speed is permanent and it is not due to browser restart
Type louder, please.