Firefox 48 for developers

To test the latest developer features of Firefox, install Firefox Developer Edition Firefox 48 was released on August 2, 2016. This article lists key changes that are useful not only for web developers, but also Firefox and Gecko developers as well as add-on developers.

Changes for Web developers

Developer Tools


  • The <details> and <summary> elements have been activated by default on Nightly and Aurora (DevTools), but not on Beta or Release:
  • The meta attributes now also supports the no-referrer-when-downgrade and origin-when-cross-origin values (bug 1178337).


  • The calc() has been improved to be closer to the specification:
  • Our experimental implementation of CSS grids has been updated:
    • Fragmentation for grid layout has been implemented (bug 1144096).
    • [css-grid] Percentage tracks are now treated as auto if grid container size is indefinite (bug 1264607).
    • <fieldset> now supports grid and flex layouts (bug 1230207).
  • The luminance value for mask-mode has been added; the auto value has been renamed to match-source, to match the spec (bug 1228354).
  • Interpolation of clip-path basic shapes in CSS animations and transitions is now supported (bug 1110460).
  • Support for horizontal-in-vertical (tate-chu-yoko) text has been added via the all value of the text-combine-upright property (bug 1097499).
  • Support for the experimental color-adjust property has been added, allowing pages to specify that background colors and images should be printed (bug 1209273).
  • The ::first-letter pseudo-element now also matches punctuation characters of type Pd that precede or immediately follow the actual first letter; this is a new requirement of CSS Pseudo-element module level 4 (bug 1260366).
  • Several -webkit prefixed properties and values have been added for web compatibility, behind the preference layout.css.prefixes.webkit, defaulting to false:
  • The text value of background-clip is now available in all type of Firefox (and not only non-release builds) (bug 1263516).
  • The absolute value of position properties on the top layer element (bug 1236828).
  • Added an internal-only syntax for @supports to detect pref (bug 1259889).


New APIs

Deprecations and removals



Canvas 2D




Changes for add-on and Mozilla developers

Older versions