You can use adblocker to do that.

I don't know how to use adblocker to prevent these. The adblocker I found requests specific words or phrases to block. The ads we are experiencing are shown one at a time in an off-yellow colored line before the first line of our inbox. They do not delete...only disappear and another totally different ad takes it's place in a long rotation of multiple ads.

Here you can install this add-on from Mozilla Add-ons.

https://addons.mozilla.org/en-US/firefox/addon/adblock-plus/

This will block the in video ads on you tube also, have fun ;)