You’re quoting about making a new profile, but have you tried a new user? Not a new Firefox profile a new system user with its own home directory that will be completely empty.
Sorry, misunderstanding. The installation is only 3 days old and on another computer with Manjaro there is the same problem with Firefox. As soon as I have time, I will create a new user profile, thanks for the hint.
Bad 
I’ve tried Plasma 6.1 on both Endeavour OS and Manjaro, and have the exact the same problem with Firefox. So far, I’ve tried many methods but only maximized window got remembered. I’ll try Neon to see if it’s a problem with Plasma or just Arch based distros.
Note:
I just tried Firefox on KDE Neon (Wayland) and it always reopened in the size of the last window I closed. So the problem is probably only related to Arch based distros.
More Note:
I did additional tests. The problem only happens when fractional scaling is used under Wayland. For my laptop, the scaling is set to 125% by default. If I change it back to 100%, Firefox works properly with window size. X11 doesn’t have this problem, maybe due to it doesn’t support fractional scaling.
Firefox on KDE Neon is OK with fractional scaling under Wayland though.