It's not hidden or anything. You just have to know how to get at it.
In fact, with the exception of binary blobs in some larger extensions, you can root around in the code of most add-ons rather easily.
If and only if you have Flagfox installed, you can dump any of the following lines into your address bar to view the source directly:
chrome://flagfox/content/flagfox.js
chrome://flagfox/content/ipdb.js
chrome://flagfox/content/context.js
If you're using Firefox 3.0, you can even do this:
view-source:chrome://flagfox/content/overlay.xul
(Firefox 2 doesn't seem to want to show it correctly)
You can do this with any extension files, if you know where they are.
chrome://flagfox/content/logo-large.png
chrome://flagfox/content/icons/flagset1/us.png
I'd have put in direct links, but this forum doesn't like chrome URIs.
I don't know of a direct way to get a directory listing off-hand, but these steps worked fine for me:
1) Enter this into the address bar: chrome://flagfox/content/
2) You will be redirected to a long URL inside the JAR, which should come up as an error.
3) Remove "flagfox.xul" from the end of the URL and hit GO again.
4) You should now be at a directory listing for jar:file:///<path to your profile>/extensions/{1018e4d6-728f-4b20-ad56-37578a4de76b}/chrome/flagfox.jar!/content/flagfox/
(assuming Firefox 3 for the above)
Just downloading the XPI and unzipping is really the best way to go, though.