The latest stable releases of Firefox, Chrome and Opera are supported.
Firefox: install Scriptish, restart Firefox, and install 4chan X.
If you get a security error involving localStorage (check the web console with Ctrl + Shift + K), go to about:config and enable dom.storage.enabled.
Chrome: Install 4chan X, that's it.
Opera: Save 4chan X into your userscripts folder. If you didn't set it already, select a folder with: Ctrl + F12 (Preferences) > Advanced > Content > Javacript Options > user Javascript Folder.
For linkification / youtube embedding, check out YouLink, which works on the entire web, not just 4chan.
For exhentai sauce, check out ExSauce.