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How do I find what the Firefox debugger's exception refers to and on which line number?

  • 12 பதிலளிப்புகள்
  • 5 இந்த பிரச்னைகள் உள்ளது
  • Last reply by jscher2000

I am using the debugger in Firefox 25.0.1 to debug a web site I am building. I don't know how to learn what the exception is when the debugger throws one. I'd like to know what the error is and where it occurred.

Thank you, Baruch

All Replies (12)

Hello 7thRay,

Thank you for contacting Mozilla Support. For questions and help with Firefox's Web Developer tools I would suggest taking a look at this page on MDN.

Please let us know if this solves your issue.

Cheers, Patrick

Patrick, Thank you for responding. The link you provided is for the documentation on the Developer Tools. However, I could not find, anywhere in that documentation, how to understand the right-most panel in the debugger that appears when an exception is thrown. Please see the attached image.

Where does the actual exception error message get shown? Where does the file name and error line number get shown?

Thank you, Baruch

Happy New Year Patrick,

It's been a couple of weeks since I last wrote. Have you been able to come up with a more definitive answer to my original question?


For getting the best results in the Web Developer Tools you can consider to install the current Nightly build alongside of the current release via a custom install to get the latest version of the Debugger.

Note that the Nightly and Aurora builds get a daily update and can be unstable at times.

Make sure not to start Firefox after the installation has finished and use the Profile Manager to create a new (separate) profile for each Firefox version.


You can use the -no-remote command line switch to open another Firefox instance with its own profile and run multiple Firefox instances simultaneously.


Thank you for writing. However, you, like Patrick, gave me a shot-gun approach to an answer, obviously not having even looked at the Firefox Debugger in action.

Will someone from this company please make an effort to answer my question posted on 12/16/13, and expanded upon and on the 18th of that month?

Thank you in advance for your help, Baruch

The current Nightly build has at least a setting to pause on exceptions and ignore caught exceptions.

I don't have code to check how it works with exceptions, so I can't confirm if it will work as you requested.


Gosh guys, I wouldn't expect you to solve this specific exception, of course you don't have the code.

Somewhere, in all of the Firefox domain, is there documentation on how to find out, in the Debugger, what the exception is, where does the actual exception error message get shown, and where does the file name and error line number that the exception occurred get shown?

I've got to use Chrome to do any debugging because that browser tells me this information. All I can find in the Firefox Debugger is that an exception was thrown.


Exception shows up in the object scope (see attached image). We'll make sure to make this more clear in the documentation.

Hope that helps, -dave


I appreciate your post. Did you see the image I attached to my post on 12/18? Your image basically duplicated mine. I know where the exception shows up, my image is supposed to indicate that.

I do not know, nor know where in the vast amount of information that the debugger provide, the following: 1) what is the exception? 2) where does it occur?

The image I'm attaching to this post, shows the Chrome Debugger in action. Please note that it provides the information I'm looking for, right up front.

Where does Firefox's Debugger provide the answers to the two questions above?


I'm sorry, I totally did miss that screenshot!

I dug a bit deeper, and it looks like our exception line number support was indeed not working right until Firefox 27 (currently in beta). Sadly I don't have a good workaround for Firefox 26.

As cor-el pointed pointed out, we encourage developers to install our Aurora release, because it gets this sort of fix more quickly than the released version. Regardless, I apologize for the 26 release's confusing (wrong) behavior here.

Thanks, -dave

Right on Dave! Finally, someone who actually looked into the issue. I thank you for that. If I understand you, the version of Firefox I'm using, 26.0, doesn't have the features built into the Debugger to provide the answers to my two questions, and that the next version, 27.0, will support that feature. Is this correct? I prefer to wait until a stable version is out; I've got enough bugs of my own to deal with, without having to contend with yours:) Thank you again for your attention to this. It is good to have a definitive answer. I will eagerly await the next version. Now, I'm working on the 3 browsers' take to the same layout code. Baruch

If you can't install Firefox 27, can you cross reference the Browser Console (Ctrl+Shift+j) to get the error description and line number? Or perhaps the two tools shows different things...