Question

I’m new to Firefox extension development, friendly speaking it’s first day, I had followed the following article in order to start with Firefox extension development: How to develop a Firefox extension.

If I download the example from above given link’s tutorial and try to install, it is installed perfectly but, when I update that example (just updating its name to the new extension name, where it is necessary in all files), and then install, I get the following message: “X could not be installed because it is not compatible with Firefox 15.0.1.” (where X is the name of my extension).

Strange behavior same code one is installing fine and other one giving error.

Any idea where it is going wrong and what is causing this?

Answer 1

The problem is that you forgot to adjust the compatibility range in install.rdf so em:maxVersion is still saying 3.1b2 (meaning Firefox 3.1b2 – a rather old version). You should change that into 15.* if you want to support the current stable Firefox version or 16.* if you want to support the current beta versions. You can also change em:minVersion into something more meaningful: I doubt that you will verify whether your extension still works with Firefox 2.0.

As to why the extension installs if you don’t change the extension ID, this is more complicated. First of all, there is a soft override mechanism for the compatibility ranges which allows extension authors to change the compatible version range without releasing a new version. In this particular case Firefox will check for extension updates by requesting this address or a similar one. The update server tells it that version 1.0 is still current but adjusts maxVersion to Firefox 8.0 – this value will override whatever is specified in install.rdf.

But that’s not the end of the story. The compatibility check mechanism was changed significantly in Firefox 10, the default assumption is now that the add-on is compatible with newer versions even if that isn’t specified explicitly anywhere. The requirements are that the extension doesn’t opt out of lax compatibility checking, doesn’t contain binary components and is compatible with at least Firefox 4. The original extension is compatible with Firefox 8 thanks to the soft override, with a different extension ID it is only compatible with Firefox 3.1b2 however (it isn’t hosted on addons.mozilla.org so the update server doesn’t know about it) which means that the lax compatibility checking doesn’t apply to it.

For reference: the behavior described in the previous paragraph is controlled by the extensions.strictCompatibility, extensions.minCompatibleAppVersion and extensions.minCompatiblePlatformVersion preferences.