It is not surprising why people do not discover this feature. I hereby argue that people are surprised to find this (or find out about this from others) because it is poorly discoverable.

Firstly, tabs existed before browsers. They appear in traditional user interfaces like control panels, where many categories of settings must be multiplexed onto one window. Tabs are obviously based on the file cabinet with dividers that have tabs sticking out.

In a traditional tabbed dialog, we don't think of a tab as being selected in the sense that files in a file manager are selected, or objects in a vector program are selected. The tab is just current. It's in the foreground, raised above the other tabs.

Furthermore, there is always a current tab.

Furthermore, you usually cannot create new tabs in a control panel dialog, nor delete them. In more modern UIs, you may be able to rearrange the order. The manipulation is limited.

Tabs are more like child windows that are multiplexed onto the same rectangular area. You don't think of selecting two tabs any more than of selecting two windows. While the browser itself has the UI focus, I cannot Ctrl-Click on another application window to select them together.

And look, in fact, you don't see the selection state change in Firefox until you select two or more tabs!

Only when you select two or more tabs do you see an additional visual state change indicating that a selection is in place, namely additional highlighting of all the selected tabs via a bright rectangle around each of their headings.

The currently shown tab is still special, independently of the selection.

Therefore, in fact, I would say that you can only select multiple tabs in Firefox. You cannot select a single tab. A selection state does not exist until you select multiple tabs.

You cannot get the additional selection state rectangle to show around just the current tab if you Ctrl-Click on it, so it's as if there does not exist a single tab selection.

Thus it's as if people are surprised that you can select tabs at all.

When I discovered this, that was my thought. Oh ..., if you Ctrl-Click or Shift-Click, then the concept of the current tab morphs to a selection paradigm. Well, that's a hack!

Another consideration is that browser does not have a separate toolbox that operates on selections of tabs. For instance, suppose we want to delete a tab. We can use the X or we can use a context menu item. That context menu item is on that tab, though. It's not a separate tool. You right click on the tab and see a Close Tab command. Now when you make a selection (two or more tabs), that command changes! It now says Close 2 Tabs (or however many). Things you can do with tabs are attached to a context menu which is invoked on a specific tab.

In user interfaces that have tools operating on objects in a document/view, it's usually possible for not object at all to be selected. And then, if you try to use a tool like an eraser (or keyboard shortcut equivalent), you may be told "select one or more objects to delete".

So for all these reasons, it's not very discoverable.

I mean, you hardly see anyone surprised that, wow, you can select multiple objects in a vector graphics drawing program. Or multiple files in a file manager, multiple e-mails in an e-mail program, etc.