In Firefox: go to the “File” > “Page Setup” in the top menu, then select “Margin & Header/Footer” tab. In the section “Headers & Footers” set everything to “–Blank–” as shown on the screenshot below and press “OK” button.
In Microsoft Internet Explorer: go to the “File” > “Page Setup” in the top menu. In the section “Headers & Footers” set everything to “-Empty-” as shown on the screenshot below and press “OK” button.
Unfortunately, it’s impossible to disable headers and footers in Safari and Chrome.