This is an archived page. It's not actively maintained.



Support for extensions using XUL/XPCOM or the Add-on SDK was removed in Firefox 57, released November 2017. As there is no supported version of Firefox enabling these technologies, this page will be removed by December 2020.

Add-ons using the techniques described in this document are considered a legacy technology in Firefox. Don't use these techniques to develop new add-ons. Use WebExtensions instead. If you maintain an add-on which uses the techniques described here, consider migrating it to use WebExtensions.

Starting from Firefox 53, no new legacy add-ons will be accepted on (AMO) for desktop Firefox and Firefox for Android.

Starting from Firefox 57, only extensions developed using WebExtensions APIs will be supported on Desktop Firefox and Firefox for Android.

Even before Firefox 57, changes coming up in the Firefox platform will break many legacy extensions. These changes include multiprocess Firefox (e10s), sandboxing, and multiple content processes. Legacy extensions that are affected by these changes should migrate to use WebExtensions APIs if they can. See the "Compatibility Milestones" document for more information.

A wiki page containing resources, migration paths, office hours, and more, is available to help developers transition to the new technologies.



Functionality similar to Python's traceback module.


Tracebacks are stored in JSON format. The stack is represented as an array in which the most recent stack frame is the last element; each element thus represents a stack frame and has the following keys:

filename The name of the file that the stack frame takes place in.
lineNo The line number is being executed at the stack frame.
funcName The name of the function being executed at the stack frame, or null if the function is anonymous or the stack frame is being executed in a top-level script or module.

See nsIException for more information.




Attempts to extract the traceback from exception.


exception : exception
exception where exception is an nsIException.


traceback : JSON representation of the traceback or null if not found.



JSON : Returns the JSON representation of the stack at the point that this function is called.


Given a JSON representation of the stack or an exception instance, returns a formatted plain text representation of it, similar to Python's formatted stack tracebacks. If no argument is provided, the stack at the point this function is called is used.


tbOrException : object