Couldn't come up with something better for the title.
But I'm trying to explain it better (and the GIF below explains it even better graphically). So, when I open Firefox on Lubuntu, no matter if it's from the Desktop shortcut (.desktop) or from the Terminal, instead of opening the Firefox right away in maximized it first goes half the screen height and THEN maximizes. So there's some sort of a lag.
Of course I've tried setting the width and height parameters, e.g.
firefox -width 1280 -height 1024 and the "maximize" parameter
firefox --window --maximize. None of those worked.
Since Lubuntu is meant for low performance hardware, I think hardware isn't an issue. I haven't seen this problem on Ubuntu and Linux Mint (yea, I know that these are a with bit different architecture than Lubuntu), which I've both used on this computer. I switched to Lubuntu, because it's more leightweight.