SUMMARY
I noticed this weird behavior in Firefox lately. I hope I can explain it properly because English is not my main language. Anyway look at the steps to reproduce it.

STEPS TO REPRODUCE
1. Open Firefox and maximize it
2. Close it
3. Open it again
4. Right click on any part of any page
5. Right click menu appears directly under the pointer so if you click and release normally the first element in the menu will be selected
6. Click on the maximize button again, you'll notice the window appears to maximize again but in some weird glitchy way
7. Right click again, the menu appears normally
8. Close and repeat, it happens every single time

OBSERVED RESULT
It appears that when restarting Firefox if the last time it was closed while maximized, right click menus appear shifted and under the mouse pointer, causing frustration while typing. It is only fixable by maximizing the window again. It appears that the window manager does not recall Firefox's last state properly. 


SOFTWARE/OS VERSIONS
OS: KDE neon 5.18
Kernel: x86_64 Linux 5.3.0-40-generic
Shell: bash 4.4.20
Resolution: 1920x1080
DE: KDE 5.67.0 / Plasma 5.18.2
WM: KWin
GTK Theme: Breeze-Dark [GTK2/3]
Icon Theme: breeze-dark
GPU: GeForce GTX 1060


ADDITIONAL INFORMATION
Not sure if it's a bug with Firefox or the window manager. It doesn't seem to be caused by my settings because it happens on a clean firefox profile too.