Bug 1674871: Unlock fission.autostart and allow to ride the trains. r=mccr8

Differential Revision: https://phabricator.services.mozilla.com/D103829

modules/libpref/init/StaticPrefList.yaml

@@ -3661,8 +3661,6 @@
 # Callers which cannot use `useRemoteSubframes` must use
 # `Services.appinfo.fissionAutostart` or `mozilla::FissionAutostart()` to check
 # if fission is enabled by default.
-# Note: This is overridden in all.js on RELEASE_OR_BETA in order to add the
-# locked attribute.
 - name: fission.autostart
   type: bool
   value: false

modules/libpref/init/all.js

@@ -2541,19 +2541,6 @@ pref("dom.ipc.processCount.webLargeAllocation", 10);
 // Disable e10s for Gecko by default. This is overridden in firefox.js.
 pref("browser.tabs.remote.autostart", false);
 
-// Disable fission for Gecko by default. Lock it on release and beta because
-// it is not ready for use and can leak URIs to telemetry until bug 1561653 is
-// fixed.
-// IMPORTANT: This preference should *almost never* be checked directly, since
-// any session can contain a mix of Fission and non-Fission windows. Instead,
-// callers should check whether the relevant nsILoadContext has the
-// `useRemoteSubframes` flag set.
-#if defined(RELEASE_OR_BETA)
-  pref("fission.autostart", false, locked);
-#else
-  pref("fission.autostart", false);
-#endif
-
 // Whether certain properties from origin attributes should be included as part
 // of remote types. Only in effect when fission is enabled.
 pref("browser.tabs.remote.useOriginAttributesInRemoteType", false);