Asking why your offline application cache manifest doesn't work in Firefox and have no way to figure out?  Not any more.  Now you can look in to the Error Console (the old fashion one) you open with Shift-Ctrl-J and check for logs I've recently added in bug 807501.  All these logs are under the Messages section.  When an update is internally invoked, there is always at least one message related to the processed cache manifest.

When an error during cache update occurs you can now see the reason:

When all goes well, i.e. Firefox has downloaded or updated your offline cache, you get:

• Offline cache update done, URL=http://example.com/cache.manifest

And when you are sure you've changed your manifest to a new version, but offline cache doesn't update in Firefox, you can check for the following line:

• Offline cache doesn't need to update, URL=http://example.com/cache.manifest

This means the manifest hasn't changed since the last time or that you didn't serve it with Cache-control: no-cache header.