Ctrl + Click on a Firefox tab places a * on it. What does this signify?

When I do Ctrl + Click on a Firefox tab, the display in the tab includes an asterisk (*) before the page name.

What does this signify? Does this have any special significance?

This is part of the Tab Mix Plus extension.

The default defined action for Ctrl + Click is Selects the tab for merging. You can then merge tabs into one window with Ctrl + Shift + U. Tabs selected for merging are indicated with a (*) before the page name (that's an asterisk enclosed in parentheses).

Editor's note: The shortcut used to be Ctrl + Shift + M, but that now opens a firefox dev tool. After extensive online searching for the new shortcut, I had to search the source code of the extension.

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