There are many Gmail users over the internet. Most of the users doesn’t need unwanted text ads to be displayed in Gmail. Are you the one among those users? If yes, then you can use Gmail Without Ads. It’s a Greasemonkey script for Firefox that is soul responsible to remove the ads from Gmail.
After installing Gmail Without Ads, the advertisement from the right sidebar will automatically disappear. The message box will extend providing more width and visibility. It will completely remove the right sidebar. There are some good Firefox add-ons to remove the ads in Gmail. But this Greasemonkey script is one of the best out of those. Unlike other add-ons, it removes the right sidebar by increasing the width of the message box.
You can use Gmail Without Ads to remove the advertisement and increase the width of the message box. One thing you should know that, the links on the right sidebar such as Print All, Expand All, etc. will be removed too. If these links are not used by you then it’s fine to have Gmail Without Ads script.