Firefox has inconsistent behaviour when it comes to opening new tabs. Sometimes they open in the foreground (i.e. switching to it immediately) and the background (not switching and staying in the current tab).
Chromium browsers handle this a lot better than Firefox. Most of their new tabs opened from middle/CTRL clicks open in the background.
The only exception is middle/CTRL clicking to open multiple previous pages in the right click back button menu. There is also no equivalent for browser.tabs.loadDivertedInBackground, so having a Chromium browser set as the default and clicking on a link from an outside application or opening a PDF with the browser set to the default PDF reader will always bring the browser to the foreground and switched to the new tab.
This might all sound really nitpicky, but this would all massively improve the QoL for anyone who uses middle/CTRL clicking to open multiple links in new tabs. If the flaws in Chromium's "open in new tab" behaviour (detailed in the paragraph above) can also be improved upon, then this would put Firefox above Chromium when it comes to opening stuff in new tabs.