Several planned code deprecations for Firefox 70, currently available on the Nightly pre-release channel, may impact extension and theme developers. Firefox 70 will be released on October 22, 2019.

Aliased theme properties to be removed

In Firefox 65, we started deprecating the aliased theme properties accentcolor , textcolor , and headerURL . These properties will be removed in Firefox 70.

Themes listed on addons.mozilla.org (AMO) will be automatically updated to use supported properties. Most themes were updated back in April, but new themes have been created using the deprecated properties. If your theme is not listed on AMO, or if you are the developer of a dynamic theme, please update your theme’s manifest.json to use the supported properties.

For accentcolor , please use frame

, please use For headerURL , please use theme_frame

, please use For textcolor , please use tab_background_text

JavaScript deprecations

In Firefox 70, the non-standard, Firefox-specific Array generic methods introduced with JavaScript 1.6 will be considered deprecated and scheduled for removal in the near future. For more information about which generics will be removed and suggested alternatives, please see the Firefox Site Compatibility blog.