User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:78.0) Gecko/20100101 Firefox/78.0
Steps to reproduce:
Move a firefox window with many tabs from a landscape monitor to a Portrait monitor.
This move can either be manual via user interaction or it can be when the computer is plugged into an external portrait display and the window is moved automatically.
This is on Windows 10 20H2 and occured with all versions of Firefox 84 and also Firefox 85.
I only recently installed a portrait monitor so am unaware and unable to give any indications of when this might have started to happen.
Firefox does not adjust it's UI elment approriately with the tab bar and window controls enlarged and the maximise/minimise/close buttons off the screen.
The web page content then strobes this happens on normal web pages and also PDFs that.
The whole application, even Firefox windows on other screens, become unresponsive.
Firefox takes up a lot of CPU and GPU resource.
Firefox may sometimes recover and adjust to the new page after 30 seconds or so. Other times Firefox crashes or it needs to be force stopped.
The UI adjusts to the portrait display and the application continues working.