Be careful, there is some script kiddie addon's out there which really aint good for learning. I recommend first you play and use a web proxy tool like webscarab/proxomitron/burp proxy suite. Make sure you sort out your network conection to connect to the proxy first.
As for addon's I recommend these, Add n Edit Cookies, Firebug, Tamper Data, User Agent switcher and Web developer.
There was an addon called netforce I believe, I can't remember if its still going but this was handy for things like base64 encoding/decoding and other encryptions.