That is likely not a Firefox but a KWin issue.
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bug filed! :)
This won't help you, but I wanted to chime in and say that I do not experience this bug. OS is Nobara, KDE 6, FF from repo. I have a KWin command to open FF with a specific size at a specific location and can resize it just fine.
Bazzite is still on KDE 5, can't wait for the update! Will probably fix this too. Thanks for sharing though 🌻
As a workaround you can use the Super + RMB shortcut to resize any window on Plasma