1. Ghostery to block most tracking: https://www.ghostery.com/download

2. Https everywhere to use https for security whenever you can -  https://www.eff.org/https-everywhere

3. Lastpass, secure password manager unlike firefoxes password manager that just shows the passwords! – https://lastpass.com/

4. Adblock plus, because it is nicer not being sold to the whole time – https://addons.mozilla.org/en-US/firefox/addon/adblock-plus/

5. Download helper, download youtube, vimeo and other videos – https://addons.mozilla.org/en-US/firefox/addon/video-downloadhelper/?src=cb-dl-users

6. Download them all, handy for downloading all the images on a page – https://addons.mozilla.org/en-US/firefox/addon/downthemall/?src=cb-dl-users

Optional geek ones mostly for developers.

7. Firebug,  any developer debugger – https://addons.mozilla.org/en-US/firefox/addon/firebug/?src=cb-dl-users

8. Google pagespeed, useful for flagging optimisations – https://developers.google.com/speed/docs/insights/using_firefox

9. Yahoo yslow, again optimisation flagging, remember to cache - https://addons.mozilla.org/en-us/firefox/addon/yslow/

Hopefully that’s of use