New Tab bookmark toolbar messes with fixed window sizes

With "Reuse last window size when opening a new window" off, I've noticed that sometimes when I spawn a new window it will have a shorter window size than the initial window.

Steps to reproduce (on 1920 by 1080 screen, GNOME desktop):

1. Make sure "Reuse last window size when opening a new window" is off and the bookmark toolbar visibility is "Only show on new tab" (default).
2. Start Tor Browser. Don't bootstrap yet.
3. Open a new window. New window shows about:torconnect is the same size (1400 by 985).
4. Bootstrap.
5. Open a new window. New window shows about:tor or about:blank (depending on settings), and is shorter (1400 by 885).

I suspect that the key difference is that about:torconnect does not show the bookmark toolbar, whilst about:tor does. But I'm not sure how this interacts with the window size logic.

I think other events can also adjust the window size. E.g. for a new profile, I think the language notification causes the initial window to be shorter than in subsequent sessions. But these are one-offs.

This isn't a major issue since each window will load web pages without visible Letterboxing (900 and 800 inner height). But it creates a slightly inconsistent experience. I'm not sure if we want to address this (e.g. changing the bookmark toolbar visibility rules), or whether we are ok with this behaviour.

/cc @ma1 @donuts