It's normal for bookmark tab to appear, I don't know a way to turn it off.

Use Ctrl+N for new tab, not Ctrl+T.

If you specify browser --view=tab --url=about:mozilla, you will get one tab with bookmarks and one with Mozilla page. And there will be confusing transitions, as if a new window is opened, while in fact only a new tab is opened.

EDIT: As non-root, in X Terminal, command
browser --view=tab --url=about:mozilla
launches MicroB window with tabs (large blue boxes-buttons at the left of the page viewport).

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Probably, you need to close all normal MicroB windows before opening a tabbed window; I don't use MicroB at all, so it's not a problem for me.

You could also try killing all MicroB processes running because when I start MicroB from terminal and close the window, terminal says it's running and I need to press Ctrl+C in terminal.
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I don't know what else to say. It-works-for-me...

When running
browser --view=tab
instead of pressing Ctrl+N, try clicking a bookmark from the list. For unknown reason, Ctrl+N doesn't work here.

I opened normal MicroB windows from desktop icon, and then one tabbed from X Terminal, and it works.
browser --view=tab --url=about:mozilla