I write to advocate for a UI option which is more compact and information dense. Similar to the 2020s Firefox designs.
Browsers today moved away from this type of utilitarian design. Even Firefox! I have to configure a UI hack to restore the "old" design:

Here's what I like most about the screenshot:
- Definitive separators for content - Tabs have a subtle but definitive separator. I can easily see where my content is at a glance without thinking.
- Minimal margin and padding - I can quickly see, understand, and operate on a large amount of context. Less mouse movement and less thinking is involved.
- Elements & content don't hide - Elements aren't hidden (like the tab close button). I don't have to think about closing a tab, and there are less steps involved.
- Subtle color usage - The active tab has a subtle blue border. That's about as much color as this UI offers. The lack of color and drop shadow lets me quickly find the active tab when I want to, and mentally filter it when I don't.
- Tabs on top - Tabs being on top reinforces a mental model of the tab being the context, not the app. This might be the most controversial suggestion.. I know this comes with several UX problems itself (window management, logical grouping of tabs to the content pane, etc)
- Customization - I have a fair bit of customization in this UI I can control. I especially like that I can add dividers to my bookmark items to create my own concept of groups.
I might just be old and stuck in my ways, so I don't know how popular this request is. But I won't use Orion without bridging the gap between the two UIs. It'd be great to swap because I'd like to support the cause.
Having additional options in the compact UI settings would be great, I think.
I'd also accept a hackier workaround like the Firefox hack does with extended browser stylesheets, scripts, etc. But ideally the workaround won't require juggling scripts and stylesheets that break with every major update.
Cheers, love what the team is doing 😄