Here is a workaround, how to open a story in background with the 'o' hotkey, rather than the foreground:

- Open a new tab, enter 'about:config'
- Search for 'browser.tabs.loadDivertedInBackground'
- Double click on 'false' to set 'Value' to 'true'
- Go to NewsBlur and open a story with 'o' and see it load in the background

I suspect this is a security feature to stop websites loading pages without user knowledge. AFAIK, it never worked by default in Firefox.