I have seen animated Themes in Firefox. I have seen Web Extensions. Could they be made to work with each other? Like Forecastfox, Could it use it’s own theme, to display the temperature outside, and animate the current weather patterns, perhaps with a mini radar map? So instead of just a lame picture or animation, the theme would actually be USEFUL. It would actually save people time knowing what the weather is outside.
Since Firefox 57 – and then soon to be following in it’s footsteps, SeaMonkey – are doing away with XUL Addons, it just sounds logical, that Extensional Themes need to be a thing for Mozilla. Maybe you want to know what to see what new movies are playing where, maybe you want to make a Google Earth Theme, or the US Postal Service wants to make a interactive theme of some kind that blends with their storefront, an Amazon Prime/Alexa Theme. I could go on and on. This should exist.
Something like this has already been implemented some time ago. The concept is called “window”. You basically leave a hole in the outer walls of a building and cover it with transparent glass.
Could they be made to work with each other?
Sure. See https://developer.mozilla.org/en-US/Add-ons/WebExtensions/API/theme/update
The only major restriction I see is that all the images need to bee included in the themes archive, you won’t be able to display dynamically generated images.