This is very confusing as seen in #12974 or #12906. My suggestion would be "legacy."
),
      createSandboxExternal(defines).pipe(
        gulp.dest(MOZCENTRAL_CONTENT_DIR + "build")
      ),
      createWorkerBundle(defines).pipe(
        gulp.dest(MOZCENTRAL_CONTENT_DIR + "build")
      ),
      createWebBundle(defines).pipe(gulp.dest(MOZCENTRAL_CONTENT_DIR + "web")),
      gulp
        .src(MOZCENTRAL_COMMON_WEB_FILES, { base: "web/" })
        .pipe(gulp.dest(MOZCENTRAL_CONTENT_DIR + "web")),
      gulp
        .src(["external/bcmaps/*.bcmap", "external/bcmaps/LICENSE"], {
          base: "external/bcmaps",
        })
        .pipe(gulp.dest(MOZCENTRAL_CONTENT_DIR + "web/cmaps")),
      preprocessHTML("web/viewer.html", defines).pipe(
        gulp.dest(MOZCENTRAL_CONTENT_DIR + "web")
      ),
      preprocessCSS("web/viewer.css", "mozcentral", defines, true)
        .pipe(
          postcss([
            autoprefixer({ overrideBrowserslist: ["last 1 firefox versions"] }),
          ])
        )
        .pipe(gulp.dest(MOZCENTRAL_CONTENT_DIR + "web")),
      gulp.src("l10n/en-US/*.properties").pipe(gulp.dest(MOZCENTRAL_L10N_DIR)),
      gulp
        .src(FIREFOX_EXTENSION_DIR + "README.mozilla")
        .pipe(replace(/\bPDFJSSCRIPT_VERSION\b/g, version))