Firefox: if the install button shows you a wall of text, you need to install Scriptish.
Opera: if you see a wall of text, that's normal (lol); you have to save it into your userscripts folder. If you haven't already, select a folder with: ctrl+f12 (preferences) > advanced > content > javacript options > user javascript folder.
The latest stable releases of Firefox, Chrome and Opera are supported.
Firefox: 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.
For linkification / youtube embedding, check out YouLink, which works on the entire web, not just 4chan.
For exhentai sauce, check out ExSauce.