While not closing tag or using head / body element is a matter of taste, not using the HTML element with a proper lang attribute and missing the title element can lead to accessibility issues.
As an end user, i can say that :
a) Chrome memory usage is far less than Firefox. try 50+ tab and you'll see.
b) Chrome is faster than firefox but it's buggy when you have many tabs
c) Mozilla never try to take my personnal data