--- a/testing/mozharness/scripts/web_platform_tests.py
+++ b/testing/mozharness/scripts/web_platform_tests.py
@@ -211,20 +211,37 @@ class WebPlatformTest(TestingMixin, Merc
                           "bin/*",
                           "config/*",
                           "mozbase/*",
                           "marionette/*",
                           "tools/*",
                           "web-platform/*"],
             suite_categories=["web-platform"])
 
+    def _install_fonts(self):
+        # Ensure the Ahem font is available
+        dirs = self.query_abs_dirs()
+
+        if not sys.platform.startswith("darwin"):
+            font_path = os.path.join(os.path.dirname(self.binary_path), "fonts")
+        else:
+            font_path = os.path.join(os.path.dirname(self.binary_path), os.pardir, "Resources", "res", "fonts")
+        if not os.path.exists(font_path):
+            os.makedirs(font_path)
+        ahem_src = os.path.join(dirs["abs_wpttest_dir"], "tests", "fonts", "Ahem.ttf")
+        ahem_dest = os.path.join(font_path, "Ahem.ttf")
+        with open(ahem_src) as src, open(ahem_dest, "w") as dest:
+            dest.write(src.read())
+
     def run_tests(self):
         dirs = self.query_abs_dirs()
         cmd = self._query_cmd()
 
+        self._install_fonts()
+
         parser = StructuredOutputParser(config=self.config,
                                         log_obj=self.log_obj,
                                         log_compact=True,
                                         error_list=BaseErrorList + HarnessErrorList)
 
         env = {'MINIDUMP_SAVE_PATH': dirs['abs_blob_upload_dir']}
         env['RUST_BACKTRACE'] = '1'
 

--- a/testing/web-platform/mach_commands.py
+++ b/testing/web-platform/mach_commands.py
@@ -61,16 +61,18 @@ class WebPlatformTestsRunnerSetup(Mozbui
             if kwargs["host_cert_path"] is None:
                 kwargs["host_cert_path"] = os.path.join(here, "certs", "web-platform.test.pem")
 
         kwargs["capture_stdio"] = True
 
         if kwargs["webdriver_binary"] is None:
             kwargs["webdriver_binary"] = self.get_binary_path("geckodriver", validate_exists=False)
 
+        self.setup_fonts_firefox()
+
         kwargs = wptcommandline.check_args(kwargs)
 
     def kwargs_wptrun(self, kwargs):
         from wptrunner import wptcommandline
         here = os.path.join(self.topsrcdir, 'testing', 'web-platform')
 
         sys.path.insert(0, os.path.join(here, "tests", "tools"))
 
@@ -106,16 +108,29 @@ class WebPlatformTestsRunnerSetup(Mozbui
         dest_manifest = os.path.join(kwargs["metadata_root"], "MANIFEST.json")
 
         if not os.path.exists(dest_manifest) and os.path.exists(src_manifest):
             with open(src_manifest) as src, open(dest_manifest, "w") as dest:
                 dest.write(src.read())
 
         kwargs = wptcommandline.check_args(kwargs)
 
+    def setup_fonts_firefox(self):
+        # Ensure the Ahem font is available
+        if not sys.platform.startswith("darwin"):
+            font_path = os.path.join(os.path.dirname(self.get_binary_path()), "fonts")
+        else:
+            font_path = os.path.join(os.path.dirname(self.get_binary_path()), os.pardir, "Resources", "res", "fonts")
+        ahem_src = os.path.join(self.topsrcdir, "testing", "web-platform", "tests", "fonts", "Ahem.ttf")
+        ahem_dest = os.path.join(font_path, "Ahem.ttf")
+        if not os.path.exists(ahem_dest) and os.path.exists(ahem_src):
+            with open(ahem_src) as src, open(ahem_dest, "w") as dest:
+                dest.write(src.read())
+
+
 
 class WebPlatformTestsUpdater(MozbuildObject):
     """Update web platform tests."""
     def run_update(self, **kwargs):
         import update
         from update import updatecommandline
 
         if kwargs["config"] is None: