The New Tab Preferences menu's width doesn't adapt to the window's width

[IuliaCristescu4]

https://bugzilla.mozilla.org/show_bug.cgi?id=1410450

[Note]:

• The New Tab Preferences menu is not entirely visible at minimum window width

[Affected versions]:

• 58.0a1 (2017-10-20)
• 57.0b10 build1 (20171019140425)

[Affected platforms]:

• Windows 10 x64
• Ubuntu 16.04 x64
• Mac OS X 10.11.6

[Steps to reproduce]:

1. Launch Firefox using a clean profile
2. Access the AS New Tab page and open the New Tab Preferences menu
3. Resize the browser window to minimum width

[Expected result]:

• The New Tab Preferences menu adapt to the window's width so that all its elements are visible

[Actual result]:

• The New Tab Preferences menu is not entirely visible

[bryanbell]

Surely this can be better, but we should recognize that the browser itself isn't useful at that size. Arguably the browser min-width should be adjusted wider.

Besides this whole menu is slated to be removed and relocated into the application preferences. Its inclusion on this page was always a temporary move.

let's not fix, and instead work on its replacement.