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How many Firefox extensions do people use?

I found myself wondering what a “normal” number of XPI extensions is, and since I was working on some other analysis runs on the Mozilla Telemetry database, I took the time to run a count across one day’s data (May 29 2014). Without much further ado, here’s the median and higher percentiles of the number of active add-ons for Firefox and Fennec (Firefox for Android):

Installed add-ons, all versions/channels
Application Platform Sessions Median 75 % 90 % 95 % 99 % max
Firefox WINNT 10472433 3 5 8 11 19 273
Firefox Darwin 364473 3 6 9 12 21 117
Firefox Linux 22701 4 8 16 23 35 112
Firefox ALL 10859607 3 5 8 11 19 273
Fennec Android 735437 0 0 2 3 6 36

Of course there are lots of caveats… These are only enabled add-ons; there was a bug in my code to collect total installed add-ons, and I didn’t feel like waiting for another multi-hour run to get the total. These are only the add-ons managed by the XPI Provider, which means Extensions, Dictionaries and Themes (but not Lightweight Themes or Plugins). Desktop versions of Firefox all come with a pre-configured Default theme, but Fennec doesn’t. Telemetry counts sessions, not users, so the results are somewhat biased toward users that restart their browser more often, and for many users Telemetry is opt-in, so the results are also biased toward the sort of users who would enable it.

That said, we have a useful estimate of how large our internal data structures will get, which can help us make implementation decisions.

The Telemetry map-reduce I used for this had a couple of bugs that I worked around to get my output; because it was a one-off I didn’t bother fixing them and re-running the analysis. That said, you can see the code, map-reduce output and further summarized data at


  1. njn wrote:

    I weep for the users with over 100 add-ons.

    Tuesday, June 3, 2014 at 10:45 pm | Permalink
  2. Simon wrote:

    Just thinking on what I have installed.
    * Firebug, because while the built-in tools are now very good, there are a few areas where they’re not quite as usable as Firebug.
    * Quick Locale Switcher – for localisation testing of a particular web app I work on.
    * ReloadEvery – for auto-refreshing a page every few seconds.
    – Something I forget the name of, but which adds a structured JSON viewer.

    Wednesday, June 4, 2014 at 7:51 am | Permalink
  3. irving wrote:

    It’s hard to imagine why somebody would have so many extensions installed deliberately, but with 500 million users pretty much every “I can’t imagine that happening” scenario is happening somewhere.

    It’s also possible that there’s malware involved; I’ve seen a few examples of profiles with multiple copies of the same malware extension installed, each one using a randomized add-on ID.

    Wednesday, June 4, 2014 at 9:09 am | Permalink