Here it is described how to check if Firebug is installed by checking if an image of firebug exists: http://webdevwonders.com/detecting-firefox-add-ons/
But it seems to be a bit outdated, cause the images he uses there don't exist anymore in firebug.
the firebug chrome.manifest
looks like:
content firebug content/firebug/ contentaccessible=yes
...
but in the whole addon I only find one png now, and that is placed in the rootfolder of the addon. But some other content is accessible, for example: chrome://firebug/content/trace.js
Ho
So, in gerneral:
I program an Addon and I want to make an image ok.png
available to all javascripts in Firefox.
I added the image in the data folder and added a chrome.manifest
content response-timeout-24-hours data/
content response-timeout-24-hours data/ contentaccessible=yes
But no way to call it via a URL like
chrome://response-timeout-24-hours/data/ok.png
How do the paths belong together? which is relative to which?
I created a Bug report here.
So if you want to make your add-on detectable you need another approach:
you can use a PageMod to attach a content script that would wait for a message from your web-app and "respond" by sending another message back to your app. you would know that if you don't receive the response, your addon is not installed. check out the documentation for more details:
https://developer.mozilla.org/en-US/Add-ons/SDK/High-Level_APIs/page-mod
I used this to make my add-on detectable.