--- a/testing/mochitest/mach_commands.py
+++ b/testing/mochitest/mach_commands.py
@@ -389,16 +389,26 @@ class MachCommands(MachCommandBase):
                     unsupported.add(key)
                     continue
             elif subsuite and test.get('subsuite', '') != subsuite:
                 unsupported.add(key)
                 continue
 
             suites[key].append(test)
 
+        if ('mochitest', 'media') in suites:
+            req = os.path.join('testing', 'tools', 'websocketprocessbridge',
+                               'websocketprocessbridge_requirements.txt')
+            self.virtualenv_manager.activate()
+            self.virtualenv_manager.install_pip_requirements(req, require_hashes=False)
+
+            # sys.executable is used to start the websocketprocessbridge, though for some
+            # reason it doesn't get set when calling `activate_this.py` in the virtualenv.
+            sys.executable = self.virtualenv_manager.python_path
+
         # This is a hack to introduce an option in mach to not send
         # filtered tests to the mochitest harness. Mochitest harness will read
         # the master manifest in that case.
         if not resolve_tests:
             for flavor in flavors:
                 key = (flavor, kwargs.get('subsuite'))
                 suites[key] = []