Dear Mozilla Mirrors – Thank You

Dear Mozilla Mirror Admins,

Over the years you have served up many petabytes of data in support of the Mozilla Mission, to make the web a free and open place for all.

It’s through your generosity that we’ve grown to a point where we are ready to shoulder most of the load of Firefox downloads on our own.


A few weeks ago we informed you of our testing of a content delivery network, or CDN, to serve Firefox binaries requested from

We set up a clear apples-to-apples “funnelcake” test, where all Firefox download requests were randomly divided into two groups: one group served through the existing mirror network, and the other served via the CDN.

The test

We setup a funnelcake experiment by having two builds, one build was seeded to existing Mirror network and the other to CDN. The two builds are identical except for the build channel names, which are reflected in the request to the “first-run” page when a user first runs the installed Firefox with a new profile. The build channels and first run page requests are listed below:

  • Mirror build
    • build channel label: release-cck-mozilla13
    • first-run page request: /firefox/14.0.1/firstrun/?f=13
  • CDN build
    • build channel label: release-cck-mozilla14
    • first-run page request: /firefox/14.0.1/firstrun/?f=14

This experiment was conducted over a 5-day period, 7/18 – 7/22.

To measure the results, we calculated the ratio of total # of first-run pages to total # of downloads for Mirror and CDN respectively.

The results

The results were conclusive. Our users, particularly in countries outside of North America and Western Europe, were significantly more likely to view the page when downloading via the CDN. This effect was seen in virtually every country we checked.

CDN powered downloads had an average install ratio 13.4% higher than Mirror powered downloads:

DateMirror Install::Download ratioCDN Install::Download ratioImprovement
Overall CDN Improvement 0.1341

It is also likely that the CDN also contributed to Firefox 14.0.1 being adopted more quickly by our user base than other recent releases.

The decision

As new web users come online around the world, often using slower networks, it is important that we do what we can to ensure they have the best possible experience — obtaining Firefox fully and quickly 100% of the time.

Because using the CDN will allow us to more easily make some Firefox changes to deliver updates more quickly and successfully in the future, we have decided to increasingly shift our download traffic to the CDN. While you should start to see the impact of this change immediately, it does not mean traffic will completely disappear.  Due to the way Firefox and our bouncer service works, some traffic will continue to arrive.

That means we still need and value your help to accomplish our mission.

Thank you!

On behalf of everyone at Mozilla, thank you for your contribution and for helping us all get to this place.

It has been through your generosity that we have been able to grow Firefox to the point that we are now able to shoulder this burden ourselves.

If we can, we would love to help you find other ways to contribute.

1 response

  1. René Leonhardt wrote on :

    It would be great if Mozilla would also provide Metalink ( and and BitTorrent downloads, maybe similar to

    Even more perfect would be Metalink (and eventually BitTorrent) download support integrated into Firefox for the fastest download experience in any existing browser: