Generally speaking, Shift+F10 displays the context menu for the active element, but usually the tab isn't an active element so I'm not sure what you were using. (Making a tab the active element requires keyboard gynmastics such as F6 to focus the address bar and then Shift+Tab enough tabs to focus the tab.)

I was aware of Shift+F10, but that's not what I recall I was donig to get it. Perhaps I'm not recalling it right at all...

You can use the context menu key on the keyboard that is usually the third of the four keys to the right of the space bar (AltGr, r-Win (compose), context, r-Ctrl).