简而言之，这两者基本上都可以解释成“插件”的意思，add-in常见于windows之上，add-on则常见于firefox之上。以下是详细解释。

add-in的解释如下：
(1) A component you can add to a computer or other device to increase its capabilities. Add-ins can increase memory or add graphics or communications capabilities to a computer. They can come in the form of expansion boards, cartridges, or chips. The term add-in is often used instead of add-on for chips you add to a board that is already installed in a computer. In contrast, add-on almost always refers to an entire circuit board.

(2) A software program that extends the capabilities of larger programs. For example, there are many Excel add-ins designed to complement the basic functionality offered by Excel. In the Windows environment, add-ins are becoming increasingly common thanks to OLE 2.0.

(3) In Windows Home Server, add-in refers specifically to the supplemental programs that users can install to extend the capabilities of the Windows Home Server program. Most add-ins are third party or community developed.

from：http://www.webopedia.com/TERM/A/add_in.html

add-on的解释如下：
a software bundle that allows the user to extend and customize Mozilla-based applications.

from：http://en.wikipedia.org/wiki/Add-on