Mozilla wants to ship lightweight Compact themes in all the channels, they want to do this by replacing the dev theme with a compact theme with a light color scheme, and a dark color scheme. The reason for Mozilla to integrate these two themes — compact light and compact dark — is users have liked the dev edition theme because of it being dark and it also saves space on low resolution displays.
Mozilla adds two news themes to Firefox 53
The company has split dev theme into two. These were now landed in Firefox 53, enabled by default and going to ride the trains. You can able to switch to either of compact theme from about:addd-ons page or from Firefox customization mode. Australis theme will going to stay as the default theme.
Summary: Dev theme in Firefox 53 has been replaced with compact themes.
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