Mozilla announced last Friday that it plans to remove heavyweight theme support from the Firefox web browser in a bug listing on the organization's official bug tracking website.
Back then, Benjamin Smedberg, one of Mozilla's senior Engineers, mentioned that Mozilla decided to "stop support for "heavyweight" themes which can do arbitrary styling and replace chrome packages" and that the organization may simply remove support or extend lightweight themes with additional features depending on how the discussion progresses.
The negative reaction to the announcement on Bugzilla and also other sites who mentioned it caused Mozilla's Kris Maglione to emphasize that the bug listing was not "just about removing support for heavyweight themes" but also for for deciding what the future of theme support in Firefox would look like.
I would just like to point out that this bug is not just about removing support for heavyweight themes. It's also about coming up with a plan what kind of theming support we want to add to replace them.
Another Mozilla employee, Kev Needham confirmed Maglione's assessment of the situation.
The point of this bug is to start to lay the groundwork for what themes support will be in the future, with the goal of making them simpler to create and maintain. This bug isn't meant to say "we're killing Complete Themes in Firefox", it's the starting point for planning what happens with them moving forward, and scoping what we can support and how.
Maglione highlighted the reason behind the decision in the same post on Bugzilla stating that current heavyweight themes "are not sustainable".
The basic fact of the matter is this: heavyweight themes, in their current form, are not sustainable. They require a complete reimplementation of the Firefox front-end CSS for every theme. They require significant, painstaking updates for every release (which happen *much* more often now than they did when the feature was designed). They require a massive amount of energy by both Firefox developers and third-party theme developers to keep alive. Most themes fall by the wayside after a couple of years (and that's being optimistic).
Mozilla basically wants to change what complete themes can do in the same way that it plans to change what add-ons can do in Firefox. The organization announced back in August 2015 that it would deprecate XPCOM and XUL based add-ons in favor of a new Web Extensions API.
Back then it asked for user and especially add-on developer input as to what the API should offer, and it is the same this time for themes.
Several add-on developers announced afterwards that they would stop development of their add-ons for Firefox, and it is likely that the same is going to happen in regards to the few theme developers who are still working on full themes for the browser.
While it is too early to tell how theme support will look like in Firefox once Mozilla is done with the changes, it is likely that it will be still superior to Chrome theming but less powerful than before.
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