AFAIK, VO+Command+Return can be used to select multiple things, not to click. For example, in the file viewer (Finder.app, similar to Nautilus/explorer.exe), it is similar to pressing Control (win/linux) and the left mouse button when the cursor is over a file to add it to the selection.
Edit: To open a page in a new tab in a Firefox with stock VoiceOver, I need to press VO+Shift+M to open context menu (aka right-click menu), and then select an option from the list (by greping, using arrows etc.).