I wanted to know how much memory the Firefox add-ons I use take. So I created a new fresh profile and looked at « Memory in use » in « about:memory ». I get 34Mb. Here is the additional memory usage I got just after start-up for every single of these add-ons.

Adblock plus: 18Mb
Firebug: 19Mb
Noscript: 4Mb
Al in one sidebar: 2Mb
Better privacy: 2Mb
Colt: 0Mb
Download status bar: 5Mb
Executejs: 0Mb
Down them all: 4Mb
Fireftp: 1Mb
Grease monkey: 4Mb
Location bar²: 2Mb
Optimize google: 4Mb
Read it later: 4Mb
Rikaichan: 3Mb
Sqlite manager: 0Mb
Undo close tabs: 0Mb
Web developper: 8Mb
WOT: 4Mb

Please note that it was just after start-up. The usage may increase in a different way for each add-on as you use your browser. I wish I had time to perform automated tests (like activating an add-on, browse some pages and note the memory usage)…