Ensure we copy mutable kwargs when constructing browser_args.

Otherwise modifying them affects future calls to browser_kwargs.
diff --git a/tools/wptrunner/wptrunner/browsers/firefox.py b/tools/wptrunner/wptrunner/browsers/firefox.py
index f9395cc..c525926 100644
--- a/tools/wptrunner/wptrunner/browsers/firefox.py
+++ b/tools/wptrunner/wptrunner/browsers/firefox.py
@@ -107,9 +107,9 @@
 def browser_kwargs(logger, test_type, run_info_data, config, subsuite, **kwargs):
     browser_kwargs = {"binary": kwargs["binary"],
                       "webdriver_binary": kwargs["webdriver_binary"],
-                      "webdriver_args": kwargs["webdriver_args"],
+                      "webdriver_args": kwargs["webdriver_args"].copy(),
                       "prefs_root": kwargs["prefs_root"],
-                      "extra_prefs": kwargs["extra_prefs"],
+                      "extra_prefs": kwargs["extra_prefs"].copy(),
                       "test_type": test_type,
                       "debug_info": kwargs["debug_info"],
                       "symbols_path": kwargs["symbols_path"],
@@ -119,7 +119,7 @@
                       "e10s": kwargs["gecko_e10s"],
                       "enable_fission": run_info_data["fission"],
                       "stackfix_dir": kwargs["stackfix_dir"],
-                      "binary_args": kwargs["binary_args"],
+                      "binary_args": kwargs["binary_args"].copy(),
                       "timeout_multiplier": get_timeout_multiplier(test_type,
                                                                    run_info_data,
                                                                    **kwargs),
diff --git a/tools/wptrunner/wptrunner/browsers/firefox_android.py b/tools/wptrunner/wptrunner/browsers/firefox_android.py
index 40a829a..8437dd2 100644
--- a/tools/wptrunner/wptrunner/browsers/firefox_android.py
+++ b/tools/wptrunner/wptrunner/browsers/firefox_android.py
@@ -45,11 +45,11 @@
 def browser_kwargs(logger, test_type, run_info_data, config, **kwargs):
     return {"adb_binary": kwargs["adb_binary"],
             "webdriver_binary": kwargs["webdriver_binary"],
-            "webdriver_args": kwargs["webdriver_args"],
+            "webdriver_args": kwargs["webdriver_args"].copy(),
             "package_name": kwargs["package_name"],
             "device_serial": kwargs["device_serial"],
             "prefs_root": kwargs["prefs_root"],
-            "extra_prefs": kwargs["extra_prefs"],
+            "extra_prefs": kwargs["extra_prefs"].copy(),
             "test_type": test_type,
             "debug_info": kwargs["debug_info"],
             "symbols_path": kwargs["symbols_path"],
@@ -57,7 +57,7 @@
             "certutil_binary": kwargs["certutil_binary"],
             "ca_certificate_path": config.ssl_config["ca_cert_path"],
             "stackfix_dir": kwargs["stackfix_dir"],
-            "binary_args": kwargs["binary_args"],
+            "binary_args": kwargs["binary_args"].copy(),
             "timeout_multiplier": get_timeout_multiplier(test_type,
                                                          run_info_data,
                                                          **kwargs),