Oh, I see you found the culprit.Maybe compare prefs.js between working & not, & see if anything there stands out?
(PS: While I would like ALL my windows to open maximized [in both SeaMonkey / FF], & while I may open all maximized, initially, it seems that over time [& I typically always restart from Session Restore], most all windows - generally except for the window that last had focus, come up windowed :frown:.
Oh, & I've even used Nirsoft's WinLister which allows you to set windows to maximized [& more], en mass, with no long term result.)