Firefox 100 User-Agent Experiment

When Firefox’s version number hits 100 in 2022 Q1, some sites might be broken because they are confused by a three-digit version number in Firefox’s User-Agent (UA) string. The type of site breakage might be big or small and is hard to predict.

To test whether users report any webcompat problems, we are running a Firefox Nightly experiment with a treatment branch spoofing a “Firefox 100” UA string.

50% of Nightly users will get a “Firefox/100.0” UA string for four weeks. We are targeting users running Windows, macOS, or Linux. We are not testing Android or iOS at this time. Users in any locale can be enrolled because we want to test sites in all languages and geographies.

If you find a broken site, please verify that site works correctly in a recent Release channel version of Firefox (91 or 92). If the site works correctly in Firefox 91 or 92 but breaks in Firefox Nightly, please file a webcompat bug on

How to check whether you are in the experiment

The experiment will enroll people running Firefox Nightly 93 or 94. Open your Firefox’s “about:studies” page and look for an active study named “Firefox 100 User-Agent”.

To check whether you are in the experiment’s treatment branch (Firefox 100) or control branch (Firefox 93), open and look for “Firefox/100.0” or “Firefox/93.0”.

If you are not in the experiment and would like to help test “Firefox 100”, you can override the default UA string by opening about:config, creating a new string pref called “general.useragent.override”, and then setting its value to open of the following strings (depending on your OS):

Operating System general.useragent.override
32-bit Windows Mozilla/5.0 (Windows NT 10.0; rv:100.0) Gecko/20100101 Firefox/100.0
64-bit Windows Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:100.0) Gecko/20100101 Firefox/100.0
macOS Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:100.0) Gecko/20100101 Firefox/100.0
Linux Mozilla/5.0 (X11; Linux x86_64; rv:100.0) Gecko/20100101 Firefox/100.0

How do I opt out of the experiment?

Open your Firefox’s “about:studies” page and look for an active study named “Firefox 100 User-Agent”. Then click the “Remove” button.


hello why this announcement slower than on Reddit? It should be announced here before at reddit right? I mean it’s weeks apart I mean months apart (source:… like is reddit firefox official channel or discourse is official channel. every new news always gone faster to reddit 1 week or worst 1 months… like … come on guys…

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I agree with @benlimanto - this announcement should have been posted here first OR the headline should have been posted to Reddit with no details posted there, instead pointing to the post on here with the full details.

Mozilla is literally wasting infrastructure and resources by trying to keep two separate platforms in sync. Just ditch Reddit already and move here.

Also, why do I get the funny feeling I predicted a majority of software vendors rapidly changing their release cycles becoming a bad thing? Of course, I didn’t predict it in such a case like this but all of the sudden lots of software vendors and projects alike have been changing their release cycle to act like agile. I don’t get the appeal.

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