--- a/js/src/devtools/automation/autospider.py
+++ b/js/src/devtools/automation/autospider.py
@@ -219,16 +219,29 @@ if 'compiler' in variant:
     compiler = variant['compiler']
 elif platform.system() == 'Darwin':
     compiler = 'clang'
 elif platform.system() == 'Windows':
     compiler = 'cl'
 else:
     compiler = 'gcc'
 
+# Need a platform name to use as a key in variant files.
+if args.platform:
+    variant_platform = args.platform.split("-")[0]
+elif platform.system() == 'Windows':
+    variant_platform = 'win64' if word_bits == 64 else 'win32'
+elif platform.system() == 'Linux':
+    variant_platform = 'linux64' if word_bits == 64 else 'linux'
+elif platform.system() == 'Darwin':
+    variant_platform = 'macosx64'
+else:
+    variant_platform = 'other'
+
+
 info("using compiler '{}'".format(compiler))
 
 cxx = {'clang': 'clang++', 'gcc': 'g++', 'cl': 'cl'}.get(compiler)
 
 compiler_dir = env.get('GCCDIR', os.path.join(DIR.fetches, compiler))
 info("looking for compiler under {}/".format(compiler_dir))
 compiler_libdir = None
 if os.path.exists(os.path.join(compiler_dir, 'bin', compiler)):
@@ -289,16 +302,22 @@ if word_bits == 32:
         env['CXXFLAGS'] = '{0} {1}'.format(env.get('CXXFLAGS', ''), sse_flags)
 else:
     if platform.system() == 'Windows':
         CONFIGURE_ARGS += ' --target=x86_64-pc-mingw32'
 
 if platform.system() == 'Linux' and AUTOMATION:
     CONFIGURE_ARGS = '--enable-stdcxx-compat --disable-gold ' + CONFIGURE_ARGS
 
+# Override environment variant settings conditionally.
+CONFIGURE_ARGS = "{} {}".format(
+    variant.get('conditional-configure-args', {}).get(variant_platform, ''),
+    CONFIGURE_ARGS
+)
+
 # Timeouts.
 ACTIVE_PROCESSES = set()
 
 
 def killall():
     for proc in ACTIVE_PROCESSES:
         proc.kill()
     ACTIVE_PROCESSES.clear()
@@ -443,28 +462,16 @@ test_suites = set(default_test_suites)
 
 
 def normalize_tests(tests):
     if 'all' in tests:
         return default_test_suites
     return tests
 
 
-# Need a platform name to use as a key in variant files.
-if args.platform:
-    variant_platform = args.platform.split("-")[0]
-elif platform.system() == 'Windows':
-    variant_platform = 'win64' if word_bits == 64 else 'win32'
-elif platform.system() == 'Linux':
-    variant_platform = 'linux64' if word_bits == 64 else 'linux'
-elif platform.system() == 'Darwin':
-    variant_platform = 'macosx64'
-else:
-    variant_platform = 'other'
-
 # Override environment variant settings conditionally.
 for k, v in variant.get('conditional-env', {}).get(variant_platform, {}).items():
     env[k] = v.format(**REPLACEMENTS)
 
 # Skip any tests that are not run on this platform (or the 'all' platform).
 test_suites -= set(normalize_tests(variant.get('skip-tests', {}).get(variant_platform, [])))
 test_suites -= set(normalize_tests(variant.get('skip-tests', {}).get('all', [])))
 

--- a/js/src/devtools/automation/variants/plain
+++ b/js/src/devtools/automation/variants/plain
@@ -1,12 +1,16 @@
 {
     "configure-args": "--enable-rust-simd",
     "optimize": true,
+    "compiler": "clang",
     "env": {
         "JSTESTS_EXTRA_ARGS": "--jitflags=jstests"
     },
     "conditional-env": {
         "linux64": {
             "JITTEST_EXTRA_ARGS": "--run-binast"
         }
+    },
+    "conditional-configure-args": {
+        "linux64": "--enable-clang-plugin"
     }
 }