fixup! Base Browser's .mozconfigs.

Use the new flag upstream to replace the Bug 41542 commit.

browser/config/mozconfigs/base-browser

@@ -39,6 +39,10 @@ ac_add_options --disable-backgroundtasks
 # Disable telemetry
 ac_add_options MOZ_TELEMETRY_REPORTING=
 
+# Disable the creation of a <something>.default that Firefox by default creates
+# for old version that could not use dedicated profiles. See tor-browser#41542.
+ac_add_options --disable-legacy-profile-creation
+
 if test -z "$WASI_SYSROOT"; then
     ac_add_options --without-wasm-sandboxed-libraries
 fi