While the "controversy" surrounding Firefox Proton has mostly cooled down, there are still people who prefer the style of "Quantum", mostly for Accessibility reasons, and have gone on to adopt CSS as a means to make the UI more Legible.
I myself am a fan of Proton for the most part, but I'll confess that while it looks beautiful by default on one laptop with a more modern monitor, the monitor on the other suffers from being a little less legible with how subtle certain dividers and shades are. On one computer, I would certainly appreciate a different "style", while maintaining the default Proton "Style" on the other.
Styles should be considered distinct from Themes for the following reason: while Themes affect the colors of the UI, and whether the browser recognizes a light or dark theme, Styles affect the UI in more tangible ways, such as what size certain aspects are (tabs, bookmarks, buttons, etc), how they are differentiated from each other (how clear dividers are, differences in shade, etc), and how accessible they are to users (the removal of "compact" mode also comes to mind).
Instead of giving the browser a complete default UI refresh (as Proton is already an acceptable default), I propose presenting the user with the option to switch between different styles based on their preference. A good starting point might be emulating the style of older Firefox releases (such as Quantum), but really all that matters is that each style is distinct enough from each other to offer clear pros and cons that would benefit different users with different use cases.
Perhaps Styles could even be user-configurable in the same way Themes are. Smart Users can already configure CSS to their pleasing, but they can't share their Styles in a way more convenient than a Github page and installation instructions. In contrast, just as Mozilla hosts a website for installing different Add-Ons and Themes, a way to curate and share Styles would be most welcome.
Thanks for submitting an idea to the Mozilla Connect community! Your idea is now open to votes (aka kudos) and comments.
As MintMain mentioned earlier, this idea:
sounds pretty similar to your suggestion. What to you think?