--- a/testing/mozharness/scripts/desktop_unittest.py
+++ b/testing/mozharness/scripts/desktop_unittest.py
@@ -863,35 +863,46 @@ class DesktopUnittest(TestingMixin, Merc
                     env['STYLO_THREADS'] = '1'
                 else:
                     env['STYLO_THREADS'] = '4'
 
                 env = self.query_env(partial_env=env, log_level=INFO)
                 cmd_timeout = self.get_timeout_for_category(suite_category)
 
                 summary = None
+                executed_too_many_tests = False
                 for per_test_args in self.query_args(suite):
-                    if (datetime.now() - self.start_time) > max_per_test_time:
-                        # Running tests has run out of time. That is okay! Stop running
-                        # them so that a task timeout is not triggered, and so that
-                        # (partial) results are made available in a timely manner.
-                        self.info("TinderboxPrint: Running tests took too long: Not all tests "
-                                  "were executed.<br/>")
-                        # Signal per-test time exceeded, to break out of suites and
-                        # suite categories loops also.
-                        return False
-                    if executed_tests >= max_per_test_tests:
-                        # When changesets are merged between trees or many tests are
-                        # otherwise updated at once, there probably is not enough time
-                        # to run all tests, and attempting to do so may cause other
-                        # problems, such as generating too much log output.
-                        self.info("TinderboxPrint: Too many modified tests: Not all tests "
-                                  "were executed.<br/>")
-                        return False
-                    executed_tests = executed_tests + 1
+                    # Make sure baseline code coverage tests are never
+                    # skipped and that having them run has no influence
+                    # on the max number of actual tests that are to be run.
+                    is_baseline_test = 'baselinecoverage' in per_test_args[-1] \
+                                       if self.per_test_coverage else False
+                    if executed_too_many_tests and not is_baseline_test:
+                        continue
+
+                    if not is_baseline_test:
+                        if (datetime.now() - self.start_time) > max_per_test_time:
+                            # Running tests has run out of time. That is okay! Stop running
+                            # them so that a task timeout is not triggered, and so that
+                            # (partial) results are made available in a timely manner.
+                            self.info("TinderboxPrint: Running tests took too long: Not all tests "
+                                      "were executed.<br/>")
+                            # Signal per-test time exceeded, to break out of suites and
+                            # suite categories loops also.
+                            return False
+                        if executed_tests >= max_per_test_tests:
+                            # When changesets are merged between trees or many tests are
+                            # otherwise updated at once, there probably is not enough time
+                            # to run all tests, and attempting to do so may cause other
+                            # problems, such as generating too much log output.
+                            self.info("TinderboxPrint: Too many modified tests: Not all tests "
+                                      "were executed.<br/>")
+                            executed_too_many_tests = True
+
+                        executed_tests = executed_tests + 1
 
                     final_cmd = copy.copy(cmd)
                     final_cmd.extend(per_test_args)
 
                     if self.per_test_coverage:
                         gcov_dir, jsvm_dir = self.set_coverage_env(env)
 
                     return_code = self.run_command(final_cmd, cwd=dirs['abs_work_dir'],
@@ -925,16 +936,19 @@ class DesktopUnittest(TestingMixin, Merc
                     parser.append_tinderboxprint_line(suite_name)
 
                     self.buildbot_status(tbpl_status, level=log_level)
                     if len(per_test_args) > 0:
                         self.log_per_test_status(per_test_args[-1], tbpl_status, log_level)
                     else:
                         self.log("The %s suite: %s ran with return status: %s" %
                                  (suite_category, suite, tbpl_status), level=log_level)
+
+                if executed_too_many_tests:
+                    return False
         else:
             self.debug('There were no suites to run for %s' % suite_category)
         return True
 
 
 # main {{{1
 if __name__ == '__main__':
     desktop_unittest = DesktopUnittest()

--- a/testing/mozharness/scripts/web_platform_tests.py
+++ b/testing/mozharness/scripts/web_platform_tests.py
@@ -333,33 +333,44 @@ class WebPlatformTest(TestingMixin, Merc
         else:
             test_types = self.config.get("test_type", [])
             suites = [None]
         for suite in suites:
             if suite:
                 test_types = [suite]
 
             summary = None
+            executed_too_many_tests = False
             for per_test_args in self.query_args(suite):
-                if (datetime.now() - start_time) > max_per_test_time:
-                    # Running tests has run out of time. That is okay! Stop running
-                    # them so that a task timeout is not triggered, and so that
-                    # (partial) results are made available in a timely manner.
-                    self.info("TinderboxPrint: Running tests took too long: Not all tests "
-                              "were executed.<br/>")
-                    return
-                if executed_tests >= max_per_test_tests:
-                    # When changesets are merged between trees or many tests are
-                    # otherwise updated at once, there probably is not enough time
-                    # to run all tests, and attempting to do so may cause other
-                    # problems, such as generating too much log output.
-                    self.info("TinderboxPrint: Too many modified tests: Not all tests "
-                              "were executed.<br/>")
-                    return
-                executed_tests = executed_tests + 1
+                # Make sure baseline code coverage tests are never
+                # skipped and that having them run has no influence
+                # on the max number of actual tests that are to be run.
+                is_baseline_test = 'baselinecoverage' in per_test_args[-1] \
+                                   if self.per_test_coverage else False
+                if executed_too_many_tests and not is_baseline_test:
+                    continue
+
+                if not is_baseline_test:
+                    if (datetime.now() - start_time) > max_per_test_time:
+                        # Running tests has run out of time. That is okay! Stop running
+                        # them so that a task timeout is not triggered, and so that
+                        # (partial) results are made available in a timely manner.
+                        self.info("TinderboxPrint: Running tests took too long: Not all tests "
+                                  "were executed.<br/>")
+                        return
+                    if executed_tests >= max_per_test_tests:
+                        # When changesets are merged between trees or many tests are
+                        # otherwise updated at once, there probably is not enough time
+                        # to run all tests, and attempting to do so may cause other
+                        # problems, such as generating too much log output.
+                        self.info("TinderboxPrint: Too many modified tests: Not all tests "
+                                  "were executed.<br/>")
+                        executed_too_many_tests = True
+
+                    executed_tests = executed_tests + 1
 
                 cmd = self._query_cmd(test_types)
                 cmd.extend(per_test_args)
 
                 if self.per_test_coverage:
                     gcov_dir, jsvm_dir = self.set_coverage_env(env)
 
                 return_code = self.run_command(cmd,