Limitations of frame scripts
Frame scripts run with system privileges and have access to the Components object, enabling them to use XPCOM objects and JSMs. Many privileged APIs will just work in a content process. Anything that just manipulates data structures will just work. XHR and Workers will work. However, some APIs that work in the chrome process will not work in a frame script. This article lists the most important of these APIs.
You should not write to or read from the disk from a frame script, in particular, the profile directory. Even if this is possible, you should not do it and may expect that it could stop working at any time. File I/O should all be done in the chrome process. For example:
Anything that tries to touch the browser UI or anything to do with XUL is likely not to work in the content process. For example:
Some services will not work in frame scripts.
Anything that needs to use Chrome windows will not work in the content process. For example:
The Places API can't be used inside a frame script. For example:
These must be registered in the content process.
window.QueryInterface(Ci.nsIInterfaceRequestor) .getInterface(Ci.nsIWebNavigation) .QueryInterface(Ci.nsIDocShellTreeItem) .rootTreeItem .QueryInterface(Ci.nsIInterfaceRequestor) .getInterface(Ci.nsIDOMWindow);
nsITabChild, that cannot be converted to an
nsIDOMWindow, so the second
getInterfacecall here will fail.
If you want a chrome window: send a message from the content process using the message manager. The
target property of the object passed into the message handler in the chrome process is the XUL
<browser> receiving the message, and you can get the chrome window from that (I'm not sure how).
By default, custom
about: pages registered using
nsIAboutModule are loaded in the chrome process. This means that you can't access their content from the content process (via XHR, for example).
There is a shim for this that makes the content of
about: pages registered by the add-on transparently available in the content process.
To avoid the shim: If you need to access the content of your about page from the content process, you need to register the
nsIAboutModule in the content process as well as the chrome process. By default,
about: pages (except for a small whitelist) are loaded in the chrome process when browsed to from the AwesomeBar.