This is useful for Firefox benchmarks (how well it handles multiple tabs via execution of a script).
- Open a new tab and type into the address bar
about:config?filter=dom.popup_maximumand press Enter. If you receive a warning, click the button that says "I'll be careful, I promise".
- Double click on the
dom.popup_maximumpreference and a popup will appear.
- Change the value to your choice (default is 20) and press OK.
More information on the preference.