Default Container Handler by Sarcastic Bird

Firefox Multi-Account Containers let you keep browsing contexts isolated, but external links (from other apps, Slack, email, etc.) open in whatever container the active workspace happens to be using. Zen Browser has no concept of a "default workspace," which makes this worse.

Default Container Handler fixes this by registering an ext+container: protocol handler. Paired with a URL rewriter like Finicky on macOS, every external link is opened in the container you designate as default (for example, "Personal"). If Zen's workspace rules then reassign the tab to a different container, the extension detects the reassignment and re-opens the link in the correct container, so focus follows the tab instead of flipping back.

Setup

Install the extension.
When prompted, select Default Container Handler as the handler for ext+container: links.
Configure Finicky (or any URL rewriter) to rewrite external links into the ext+container:name=<ContainerName>&url=<encoded-url> format.
Full setup instructions and a sample Finicky config are on GitHub: https://github.com/sarcasticbird/default-container-handler

Privacy

This extension does not collect, store, or transmit any data. It only reads tab URLs long enough to route them to the correct container, and creates containers on demand when one doesn't exist.