Tor Browser can no longer be maximized when using a custom browser homepage

When using a custom browser homepage, Tor Browser can no longer be maximized.

Tested with TBB 5.5a3.

Steps to reproduce:

• start Tor Browser
• set a custom browser homepage (using preferences or about:config browser.startup.homepage)
• restart Tor Browser
• click the window maximize button

Expected result:

Tor Browser maximization should just work like with the default about:tor homepage.

Actual result:

Tor Browser can no longer be maximized by pressing the maximization button.