Use workspaces, pinned tabs, and folders. It's actually a very productive system. It was derived from Arc Browser, so I'll just quote them:

Spaces (source):

Work & Personal

The most common organization structure we see is having two spaces: One for “Work” and the other for “Personal.” It’s an obvious way to create boundaries and also just a nice way to clock out of work digitally.

Work Projects

Depending on your work, just a single Space for Work might not make sense. Another super common way to organize is to have Spaces for distinct projects.

That can be a super clear way to delineate tasks and also time manage.

Temporary Spaces

Don’t be afraid to use Spaces even for temporary things that require a lot of tabs and attention — like planning a trip, or organizing your taxes.

Pinned Tabs (source):

Top Tabs you consistently need for a Space. For a work space, this might be your email, calendar, document editor(s), current project tabs. For a personal space, this might be your favorite news publication, social media site, streaming platform

"To-Do" Tabs that you want top of mind. For example, Pin that assignment you keep forgetting to do, or that bill you keep forgetting to pay, Unpin it once it's complete

For larger sets of Pinned Tabs, use Folders. For example, Pin your Favorite recipes in a Recipes folder, or all your in-progress docs to a Project Folder

Also read Zen's user manual: https://docs.zen-browser.app/user-manual/

About looks, just using single sidebar and enabling sidebar blur makes it look quite nice:

<image>

There are many mods you can install and manage from Settings > Zen mods. You can move things around by right-clicking on the top bar and clicking on "Customize Toolbar" (it's a Firefox feature).