Necessary downloads:
(This hack is only compatible with Firefox)
chromEdit [website] [download] (open the file with firefox)
The next thing you need to do is copy the css code I have displayed below to the userContent.css file. After you have chromedit installed, click on Tools in your Firefox browser, and then edit user files. Click on the userContent.css tab and ad the code to the file. Click save, and the gmail ads will be gone!
The pros: This hack is pretty awsome! You can view gmail like an expensive paid email service without ads and keep all the features.
The cons: I'm guessing that if everybody did this, then gmail would probably stop allowing new users or cut some of their limitations to save money. It's also very possible that this hack will not work in the future after gmail updates.