Reflections of a scattered Mind!!

"Not all scattered pieces are of broken glass"

Firefox Addons: Little Housekeeping Tips

I’m working on a Firefox addon in the last few weeks and this is my 1st experience with Firefox addons. During my tiny experience, I came across some of those little things that helps you when you’re in trouble & here I’m writing about those simple tips. Of course, I mixed some changes in Gecko 2.0 also along with these….

I know these are very basic, and almost everyone who worked on the addons might be knowing them, but hey, didn’t I mention you that this is my first encounter with building firefox addon?

How can I be sure if a Binary XPCOM component is loaded / attempted to load from Firefox?

Close all Firefox windows and run the following from command line:

set NSPR_LOG_MODULES=all:5  
set NSPR_LOG_FILE=c:\ff.log  
C:\Program Files\Mozilla Firefox\firefox.exe  

What does it do? Line 1 sets the modules to load & the log level. In the above commands, we are asking to log all modules with log level 5 (meaning all logs). Line 2 sets the path to log file. Now, just launch the Firefox from command line (so that the variables are set)

Troubleshooting Tip: Do not use quotes around the log file name, like set NSPR_LOG_FILE="C:\ff.log". I tried it the 1st time and it doesn’t work.

References: Troubleshooting XPCOM components registration, NSPR Logging

How can I sure if an XPCOM component is successfully registered?

This works for both binary & JS components.

Enable Chrome errors in Error Console, if you haven’t done already! To enable, set javascript.options.showInConsole to true in about:config preferences.

Open Error Console (Ctrl+Shift+J)

Evaluate Components.classes["components-contract-id"].name. Use with the components’ contract id.

If it gives no error, the component is successfully registered.

References:  javascript.options.showInConsole, Troubleshooting XPCOM components registration

How to log errors from the XPCOM components?

This is simple thing, and I realized a bit late about it. This is currently how I’m debugging my components (need to see if there is a better way). You can report error messages to the Error Console with:

Components.utils.reportError("This is the description of the error");

References: Error Console


The inconvenience with the reportError API is that it logs all messages as errors. To log at various levels (debug / warning / info), we can use the nsIConsoleService. The logStringMessage() function of this service looks very handy.

I haven’t used it yet, so I cannot give any more information on this!

References: nsIConsoleService Reference

UnPack option

If you are using binary components in the addon, you MUST set the unpack value to true in install.rdf file.

References: unpack in Install Manifests

Bootstrap Addons

This feature introduced in Gecko 2.0 answers one of my long-time questions – Why should I restart the browser everytime I add / update / disable / remove an addon?

Though this feature is good, it looks complicated to do all the manual work. I’ve skipped trying this feature for now, but I’ll look into this some other time.

References: Bootstrapped Extensions

Gecko 2.0 XPCOM Changes

Though there are many changes, I’m listing the few which I came across.

  • Javascript component must export NSGetFactory() instead of NSGetModule() function. Binary component must export NSModule() instead of NSGetModule() function. I’ve explained this in detail in the next section.
  • Components in components/ folder are no longer auto-registered & auto-loaded.Components registration will be done through the manifest file.
  • Category Registrations are done through manifest file. Also, category names are modified.
  • You’ll have to use the XULRunner SDK 2.0 for Gecko 2.0

Troubleshooting Tip: Use the sample CPP file (from the references) to get started for binary components. I got a few compilation errors initially, and then I has to add MOZ_NO_MOZALLOC to preprocessor definitions as read somewhere, and it worked..

References: XPCOM Changes in Gecko 2.0, nsSampleModule.cpp source code

Component Registration in Firefox 4.0

All the components to be registered has to mentioned in the chrome.manifest file.

  • All the interfaces (xpt files) are to be registered using interfaces command.
  • A JS component must use component command. It needs to mention its class id also. The contract id can be assigned to this JS component by a separate contract command.
  • A binary component can be registered with binary-component command. A binary component registration need not specify any class-id or contract-id in the manifest file, since they are available through the exported NSModule() function.

References: Chrome Registration

If you have felt all the content is copied from somewhere, its ok because its true. I’ve learned many things from MDC itself, I’m just aggregating those things here!!

If I’ve got something wrong in here, please do mention them in the comments as I have to correct them in the post as well as in my mind…