Mobile polish for new home page

[Sancus]

There are a few issues on mobile and it needs a polish pass:

• The site's minimum width is too wide, some phone sizes will cut text off or show horizontal scrollbars, especially in non-English languages.
  

• Spacing between sections can take up an entire screen on mobile, compared to 40-50% at most on desktop. This should be more consistent, no benefit to making people scroll an empty screen.
  

• At smaller screen widths, sometimes the download buttons visually break apart.
  

(all screenshots are from current Firefox Beta on Android)