diff --git a/toolkit/components/featuregates/Features.toml b/toolkit/components/featuregates/Features.toml
index c53d097ae5e6..a90494490d2c 100644
--- a/toolkit/components/featuregates/Features.toml
+++ b/toolkit/components/featuregates/Features.toml
@@ -132,3 +132,13 @@ type = "boolean"
 bug-numbers = [1673971]
 is-public = true
 default-value = false
+
+[firefox-100]
+title = "experimental-features-firefox-100"
+description = "experimental-features-firefox-100-description"
+restart-required = false
+preference = "general.useragent.forceVersion100"
+type = "boolean"
+bug-numbers = [1748798]
+is-public = true
+default-value = false
diff --git a/toolkit/locales/en-US/toolkit/featuregates/features.ftl b/toolkit/locales/en-US/toolkit/featuregates/features.ftl
index c413d062aaa8..31c446c8c88b 100644
--- a/toolkit/locales/en-US/toolkit/featuregates/features.ftl
+++ b/toolkit/locales/en-US/toolkit/featuregates/features.ftl
@@ -77,3 +77,8 @@ experimental-features-js-warp-description = Enable Warp, a project to improve Ja
 experimental-features-ime-search =
     .label = Address Bar: show results during IME composition
 experimental-features-ime-search-description = An IME (Input Method Editor) is a tool that allows you to enter complex symbols, such as those used in East Asian or Indic written languages, using a standard keyboard. Enabling this experiment will keep the address bar panel open, showing search results and suggestions, while using IME to input text. Note that the IME might display a panel that covers the address bar results, therefore this preference is only suggested for IME not using this type of panel.
+
+# Firefox 100
+experimental-features-firefox-100 =
+    .label = { -brand-product-name } 100 User-Agent String
+experimental-features-firefox-100-description = Make { -brand-short-name } send websites a User-Agent string that pretends to be { -brand-product-name } version 100. Use this setting to test whether websites will break when { -brand-short-name } hits a three-digit version number. The real { -brand-product-name } 100 is scheduled to be released in May 2022, so start testing your websites now!