I think I have found an answer, but it seems like a bug to me.

If I go into "Customize" from the menu and toggle "Title Bar" (turning it white) the new windows will open maximized. If Title Bar is grayed out the new windows will not open maximized.

I have similar problem on a Mac only I want it to be the other way. I want FF to open on the last remembered size instead of maximized. Currently it's basically doing what ever it wants. Sometimes it will open maximized, sometimes not. It's very annoying.

Here's the question I posted, maybe the answer to your question might as well be there...

https://support.mozilla.org/en-US/questions/1007659?esab=a&s=&r=0&as=s

Start Firefox in Safe Mode {web link} While you are in safe mode; Press the <Alt> or <F10> key to bring up the tool bar. Followed by;

Windows; Tools > Options Linux; Edit > Preferences Mac; application name > Preferences

Then Advanced > General. Look for and turn off Use Hardware Acceleration.

Poke around safe web sites and see if there is still a problem. Then restart.

If you look at my previous post I did find the answer.

Toggling Title Bar in Customize causes windows to fail to open in maximized mode. I have confirmed this on 2 different computers, both running Firefox 30. One of them is an old XP machine while the other is running Windows 7.

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