Gmelius is a cross browser (Opera,Firefox,Chrome,IE) extension to enhance your Gmail interface, after installing it you will be presented with a long list of settings with check boxes to easily decide what your webmail should look like. Other customizable options are removing chat and status of chat contacts, colourize navigation icons, remove People Widget, make header autoexpandable, apply the same font to all inbox messages and add attachment icons, to tweak these settings you just need to check or uncheck a tickbox.
This extension will not stop Google email scanning your messages but by removing the advertisements, besides getting a better Gmail experience, you can get back at Google by depriving them of revenue earned invading your personal privacy. Nearly all free email services display advertisements of some kind but only the most busybody services like Gmail go to the extra length of scanning people’s personal messages.
If you care about privacy it’s best to use a different email service but if you are going to use Gmail because it has features you can not find elsewhere and you are not encrypting your messages removing advertisement will send Google a strong message about how relevant people think their adverts are.