Unique fingerprint with the screen size
According to EFF Fingerprinting test, the reported size is no longer unique. With my VirtualBox setup and a screen of 1920x1080, the reported size is 994x900 (see tails.png attachment).
You can see that the missing 6px are on the left of the browser in red.
Since Tails use Gnome, I tried with Ubuntu 18.04 in a VM, but the issue does not appear (see gnome.png attachment). I also tried Awesome WM natively (see awesome wm.png), and no problem. I’m going to test on a Debian gnome+nonfree.
These elements seem to show that this is a bug related a priori to Tails and not Tor Browser. I’m not totally sure because on Reddit, a guy seem to have a problem, this time with the height and not the width: https://www.reddit.com/r/TOR/comments/9dn9ud/tbb_80_messes_up_screen_size/
One of the picture in the news about Tails 3.9 release, also show this extra left margin (which is by the way, not set in the default CSS of all pages).