I try to show you and coment your post:
a) everything showed here is only for FF. It happens on normal mode, but ALSO IN INCOGNITO. BUT ONLY AT EXISTING ACCOUNT, AS SHOWN IN VIDEO.
b) during video, all extensions are disabled, except of plugins like codecs, flashplayer, shockwave, silverlight and stuff…
c) Cleared page cache, even sacrificed all other browser data and ran ccleander to clean everything I could
d) will not change my resolution. It seems to have nothing to do with it (hard to see from video, but in standalone client, while fullscreen, everything is sharp. While in browser in normal mode, everything is sharp. While in browser in F11 fullscreen, everything is sharp. Only this, auto fullscreen mode, there are things little bit blurry)
e) my resolution is 1920x1080
f) FF version 55.0.1 x64
g) this wierd auto fullscreen triggers automatically, only in FF and only at existing account. free temp accounts are ok
h) during this fullscreen mode, I am not able to show console. Not even by key shortcut. This is the same fullscreen, as you run any youtube video on fullscreen.
Note: This video is made from 3 separate tracks, because fraps refused working on win10 and native win10 capture app stopped recording everytime I entered the fullscreen mode (probably because it is designed to record games and as non-fulscreen it takes like browser, the fullscreen it takes like the plugin itself and it thinks that the browser has ended … blah, cannot explain it … simply said, I was not possible to record it in one track. So the transitions in video are not fluent, there are sharp jumps, but the “workflow” of app is shown exactly how it is)
1st step - login with temp account. No fullscreen activated.
2nd step - logout and login your account. When you are logged in, The first click inside the game activates this fullscreen and shows the message of entering fullscreen.