You may want to install Stylish, create an empty stylesheet with sth like

  html * {line-height:170%;}

and enable it when needed - that's what I do from time to time; I also have one with

  html * {color:black !important;}

and one with

  html body, 
  html div,
  html span,
  html p 
   {font-family:Georgia !important;}

With this, tightly packed gray-on-gray websites with unreadable typeface no longer bother me.