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Why don't all of the new tabs open up when I choose "Open selected items"?
There is a feature on the search results pages that allows you select a bunch of items, and have up to 20 of them open in new browser windows.
Browsers have a feature that stops exactly this behaviour because some websites will open tabs and/or windows willy-nilly, and it's a real problem trying to close them. Mostly, it's advertising, or some sort of scam. So how can you tell your browser that this behaviour is allowed?
Try opening the set of windows, and if it doesn't work look at the end of the address bar. You may see something like this:
It's the window icon with the red ❌ that's the problem here. Click it and something like this should pop up:
Choose Always allow pop-ups from mvow.com.au and click Finished. Next time you try, you should get all the tabs you were looking for.
Opening many new tabs in a short period of time is hard on the server and your browser.