>>6718828If you navigate to chrome://browser/content/browser.xul and open the inspector (Ctrl-Shift-I) you can inspect the button and see that it's declared in the HTML with class "tabs-newtab-button". You can also see all the rules applied in the right pane, including stuff it inherited. Struckout lines means they were overridden.
The CSS selector for a class is .class-name, so use .tabs-newtab-button.
>>6719016pic related