Firefox extensions priority

Question

It's known fact that Firefox has no permissions mechanism for extensions. So nobody know (including Firefox) what extension actually doing, it can do everything that it wants (of course within api).

The question is about extensions priority. Imagine that user installs several New tab extensions. He restarts Firefox, what New tab will work? I was trying to figure it out.

Firefox data is stored in C:\Users\UserName\AppData\Roaming\Mozilla\Firefox\Profiles\Whatever.default, also there are extensions directory which contains extensions data actually. Some data about all installed extensions is stored in addons.json and extensions.json.

First of all, installation order doesn't play any role. Extension that was installed earlier can "beat" extension that was installed later. And vice versa.

Also I was trying to enable/disable extensions, look at last changed files and compare them. Nothing usefefull. Just boolean flags, that extension is enabled or disabled and is it enabled or disabled by user.

Also I tried to change extensions order in addons.json and extensions.json, but it seems it may not affect anything.

Most of extensions, that use "low api" use similar code for new tab:

var newtab = {
    init: function ()
    {
        gBrowser.addEventListener("NewTab", newtab.opentab, false);
    },

    opentab: function (aEvent)
    {
        // action here
    }
}

window.addEventListener( "load", newtab.init, false);

Some references:

Firefox documentation. On page load. "Low" api style.

Firefox documentation. sdk/tabs module. "High" api style.

So, how do you think Firefox's extensions priority works?

Are all extension receiving events (that new tab is opened – gBrowser "NewTab" event in low level api, onLoad/onReady/onActivate in sdk/tabs and others)? And how Firefox decide whose callback to process? Or Firefox processing all callbacks and the result of last one is displaying? If that was true, I think there were races between different extensions and sometime there will be different new tabs. But there are always stably one new tab extension is working.

Would appreciate for any thoughts.

nmaier nmaier · Accepted Answer · 2014-07-01T18:46:10

There is not reliable, obvious "order". It depends on a lot of factors:

Order of initialization, which is dependent on the order of how the add-on manager stored installed extension internally, on the add-on type (restartless add-on start up at another point than non-restartless add-ons do).
Order/time of the actions of the add-ons itself. Does an add-on immediately modify something? After a timeout? After watching a window? After some other asynchronous API callback (e.g. after querying it's own meta-data from the add-on manager)
Order and position of registered events.
In most "API"s, the later add-on overrides the earlier one.
In other APIs, e.g. the DOM API, the first add-on may win (e.g. by canceling the propagation of a DOM event).

So, there essentially is no reliable way an add-on author could really, reliably influence things in her favor (too much implementation details at play).

And add-ons will trying to modify a limited resource (e.g. the new tab page, which is a simple preference really; there can be only one), will essentially fight with each other.

Solution?

However, most of the time, it would be possible for an add-on to detect that there is something going on (e.g. detect the newtab page was already overridden, or was overridden again later) and tell the user that there might be a problem with another add-on interfering. ("Dear friend; we detected you already have a new tab add-on; please disable the other add-on in order to have NewTabX function properly.")

Problem?

For many add-ons, this is not really a problem, as they don't really work with limited resources, or there are explicit ways in Firefox for the user to choose what extension to use (e.g. you there is no problem when two add-ons add a context menu for the same webpage link, the user would just see two items to choose from; "Download in Add-on ABC"; "Download in Add-on XYZ");

Of course, when there is a truly limited resource, like the one and only newtab page, then you better not install multiple newtab-page add-ons, or things may go crazy.

Subquestions

Are all extension receiving events (that new tab is opened – gBrowser "NewTab" event in low level api, onLoad/onReady/onActivate in sdk/tabs and others)?

With usual addEventListener stuff and SDK APIs: Yupp. If two things override the same onxzy= Element attribute in the Browser DOM, then of course not; the latter override wins.

And how Firefox decide whose callback to process?

Normal addEventListener order per the DOM specs. Or whatever a non-DOM API uses (FIFO, LIFO).

Or Firefox processing all callbacks and the result of last one is displaying?

If multiple add-ons modify the same thing from a callback, the last one wins. However, quite often, add-ons will do different things when reacting to the same event (e.g. on tab load Greasemonkey may load user scripts, Adblock Plus will block certain elements, Linkify will linkify text links, etc.)

That's also the reason all events have to process, and Firefox cannot and will not simply pick something and ignore others.

If that was true, I think there were races between different extensions and sometime there will be different new tabs. But there are always stably one new tab extension is working.

Yep.

Comparison to the extension systems of other browsers.

Most of the time, other browsers essentially work the same as Firefox. There is some internal order that is not necessarily cast in stone or obvious, DOM events (in websites) will be dispatched to multiple add-ons, multiple add-ons can add listeners to other stuff (e.g. chrome.downloads.onCreated.addListener()) and the latest/first add-on wins, depending on the API.

Conclusion

All extensions trying to "own" a limited resource may clash with other add-ons trying to do the same. Expect the unexpected :p