why do we need wpt exrension in chrome but not in firefox

[pmeenan]

Andy is correct and the pop-up has nothing to do with your chrome getting slow. That is how the WPT chrome extension inspects network requests using the remote debugging API and the warning is there just because the API provides a lot of power and regular users wouldn't want extensions running with that amount of access to their browser.

If you remove it from the options the tests will not work. The extension also drives the browser navigation and without it Chrome would just open to a blank page, timeout and exit.