Optimize for bigger fonts in Firefox

First set zoom to  120% in Settings Zoom. Use Fixed Zoom add on for easy access.
Set fonts in Preferences, fonts and colours. Don't forget to adjust minimal font size in Advanced and untick  "Allow pages to choose their own fonts, instead of your selections above".
Make a Chrome folder in your home/username/.mozilla folder. Download template here and paste in userContent.css:
* {
  line-height: 2em !important;
}
html, body, div, section, article, p, span, th, td {
  line-height: 2em !important;
}
This helps to prevent problems with line spacing.