&gt; Part of the decision has already been made. We are moving Firefox addons (themes and extensions) away from a model where you can perform arbitrary styling or scripting of the browser chrome. This is an engineering-driven decision, and it&#x27;s unavoidable and necessary for the long-term health of Firefox. Not only are we moving Firefox away from XUL, but we are likely going to make significant changes in the way the UI is structured. <i>It is likely that some parts of the UI will be implemented using native widgets, and other parts will be implemented in HTML</i>, but the exactly DOM structure may involve independent connected with well-defined API surfaces.<p>Official statement from the Mozilla post in the discussion regarding removal of support for &quot;heavyweight&quot; themes. Emphasis mine.<p>That’s a pretty clear statement that it won’t be 100% HTML.<p>Also, the fact that &quot;arbitrary styling and scripting&quot; won’t be possible is another issue.<p>Tell me how I am supposed to write an addon that adds tab-previews as thumbnails when you hover over a tab like Vivaldi is doing it: <a href="http:&#x2F;&#x2F;i.imgur.com&#x2F;vqysJs1.png" rel="nofollow">http:&#x2F;&#x2F;i.imgur.com&#x2F;vqysJs1.png</a> ?<p>How am I supposed to write an addon that colors the navbar and the current tab in the theme color given by the HTML, or, if not existing, the favicon?<p>With current addons I can do that, with the new addon system, I’m seriously fucked.