Hamburger Menu: move-to-top-left-Linux-Windows.css half-broke in FF71
The area of the hamburger menu button now takes up the entire length of the tab bar and the address bar. With no tweaks it looks like this:
I was able to get the button "image" back into the corner by tweaking the padding/margins but the clickable button remains the same size (as illustrated in this gif):

What values have you used for padding and margins?
Sorry I took so long. I did see your reply back in February and meant to get to it later, just forgot about until I logged in just now. My margin tweak broke further with the next Firefox update, shortly after I posted this. The button image was completely hidden in the tab bar when the Firefox window was active. So I stopped using the tweak entirely. Here is what I tweaked in the CSS regardless:
:root[uidensity=compact] #PanelUI-button { margin-top: -56px; }