How to modify appearance of tabs themselves with WebExtension?

Old extensions such as the legendary TabMixPlus have had features such as tab wrapping, as seen in the attached photo. I have looked through the API list and can’t seem to find anything that can modify the actual appearance of the tabs in the browser eg. position, colour etc.

How is this done?

You can modify how the tabs look via theme API, see

However for things like multiple rows of tabs there is no official support, though you can still kind of achieve them it seems, see Tabs in two or more Rows (like TabMixPro) in Quantum?

There is no WebExtensions API for modifying toolbars in that way. You’ll need to use custom style rules in a userChrome.css file. Some resources:

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That’s great, thank you!

Do you believe that this could be done with an extension?

Some day, perhaps. But not now because the APIs have not been finalized and implemented yet.