I am using Ubuntu 14.04 and I am browsing with the newest Firefox.
Where can I get addons/plugins (for example Adblock) and how do I install them?
Just google what plugin you want and it takes you to the firefox's official webpage for addons. You can simply just download from there and it automatically recognises which os you are using, so no need to worry all that. For example, the addon Adblock is right here