firefox add-on vs. extensions vs. plugins

Question

I want to write scripts for firefox. It seems that firefox has different terms, like add-on, extensions, plugins. and I have a feeling they\'re not all the same. Can you sum

Answer 1

Augmenting the useful answer above, I found this high-level summary helpful:

Extensions differ slightly from plug-ins. Plug-ins usually have a narrow set of abilities. [..] Since plug-ins and extensions both increase the utility of the original application, Mozilla uses the term "add-on" as an inclusive category of augmentation modules that consists of plug-ins, themes, and search engines.

(from http://en.wikipedia.org/wiki/Plug-in_(computing))