Mozilla's engineers have announced the removal of Firefox complete themes as a way to lighten the browser core and remove a feature they don't see as heavily used anymore.
A few details need explaining before going forward with this story. Firefox has two sets of themes, which can be quite hard to differentiate, even if you're a Firefox aficionado.
There are complete (heavyweight) themes and Persona (light) themes. The complete themes affect everything in the browser, from colors to button sizes, from borders to tab placements. On the other side, light or Persona themes are just basic skins, something that users apply, change the background of the browser frame, but leave buttons and tabs alone.
As you can imagine, Firefox complete themes need more access to the browser core to do their magic. Because of this, Firefox needs to have the code in place to allow these themes to work.
The announcement to remove the support for complete themes was made two days ago on Firefox's bug tracker, and Mozilla engineers did not specify a date when this would happen.
What is known is that Firefox complete themes are definitely out of Firefox's future.
Mozilla staff did leave the door open for migrating some of the customization options to light themes, but only a few, the ones that deal with changing colors or icons.
The Firefox community did not respond well to this piece of news, most seeing it as the engineers "chromifying Firefox."
The change is part of Mozilla's Great-or-Dead initiative, which plans to simplify the Firefox codebase and remove features that are not popular.
Previous casualties of this initiative include the removal of Tab Groups which, according to internal Mozilla telemetry data, only 0.01% of users were using.