firefox/wrapper: Set new style override for legacy profiles & allow downgrades

While Firefox 68 started messing with our profiles and required new
profiles on binary location changes Firefox 69 now verifies that we
aren't downgrading to an older Firefox even of the same version. If you
switch between two channel versions and/or between nixpkgs releases
Firefox will refuse to start and demand a fresh profile. Disabling the
downgrade protection works around that issue.

pkgs/applications/networking/browsers/firefox/wrapper.nix

@@ -129,6 +129,8 @@ let
             --set MOZ_APP_LAUNCHER "${browserName}${nameSuffix}" \
             --set MOZ_SYSTEM_DIR "$out/lib/mozilla" \
             --set SNAP_NAME "firefox" \
+            --set MOZ_LEGACY_PROFILES 1 \
+            --set MOZ_ALLOW_DOWNGRADE 1 \
             ${lib.optionalString gdkWayland ''
               --set GDK_BACKEND "wayland" \
             ''}${lib.optionalString (browser ? gtk3)