diff --git a/python/wpt/exporter/__init__.py b/python/wpt/exporter/__init__.py
index c9b5e5d0285..b2df5675598 100644
--- a/python/wpt/exporter/__init__.py
+++ b/python/wpt/exporter/__init__.py
@@ -19,6 +19,7 @@ import dataclasses
 import json
 import logging
 import re
+import shutil
 import subprocess
 
 from typing import Callable, Optional
@@ -50,8 +51,12 @@ class LocalGitRepo:
         self.path = path
         self.sync = sync
 
+        # We pass env to subprocess, which may clobber PATH, so we need to find
+        # git in advance and run the subprocess by its absolute path.
+        self.git_path = shutil.which("git")
+
     def run_without_encoding(self, *args, env: dict = {}):
-        command_line = ["git"] + list(args)
+        command_line = [self.git_path] + list(args)
         logging.info("  → Execution (cwd='%s'): %s",
                      self.path, " ".join(command_line))
 
@@ -142,9 +147,9 @@ class WPTSync:
     suppress_force_push: bool = False
 
     def __post_init__(self):
-        self.servo = GithubRepository(self, self.servo_repo)
-        self.wpt = GithubRepository(self, self.wpt_repo)
-        self.downstream_wpt = GithubRepository(self, self.downstream_wpt_repo)
+        self.servo = GithubRepository(self, self.servo_repo, "main")
+        self.wpt = GithubRepository(self, self.wpt_repo, "master")
+        self.downstream_wpt = GithubRepository(self, self.downstream_wpt_repo, "master")
         self.local_servo_repo = LocalGitRepo(self.servo_path, self)
         self.local_wpt_repo = LocalGitRepo(self.wpt_path, self)
 
diff --git a/python/wpt/exporter/github.py b/python/wpt/exporter/github.py
index 08b491765be..824bcb997f4 100644
--- a/python/wpt/exporter/github.py
+++ b/python/wpt/exporter/github.py
@@ -55,9 +55,10 @@ class GithubRepository:
     This class allows interacting with a single GitHub repository.
     """
 
-    def __init__(self, sync: WPTSync, repo: str):
+    def __init__(self, sync: WPTSync, repo: str, default_branch: str):
         self.sync = sync
         self.repo = repo
+        self.default_branch = default_branch
         self.org = repo.split("/")[0]
         self.pulls_url = f"repos/{self.repo}/pulls"
 
@@ -105,7 +106,7 @@ class GithubRepository:
         data = {
             "title": title,
             "head": branch.get_pr_head_reference_for_repo(self),
-            "base": "main",
+            "base": self.default_branch,
             "body": body,
             "maintainer_can_modify": False,
         }
diff --git a/python/wpt/exporter/step.py b/python/wpt/exporter/step.py
index d56c4a3e852..9781353ce15 100644
--- a/python/wpt/exporter/step.py
+++ b/python/wpt/exporter/step.py
@@ -190,7 +190,7 @@ class CreateOrUpdateBranchForPRStep(Step):
             return branch_name
         finally:
             try:
-                run.sync.local_wpt_repo.run("checkout", "main")
+                run.sync.local_wpt_repo.run("checkout", "master")
                 run.sync.local_wpt_repo.run("branch", "-D", branch_name)
             except Exception:
                 pass
diff --git a/python/wpt/test.py b/python/wpt/test.py
index f30e01eb79c..742796035c6 100644
--- a/python/wpt/test.py
+++ b/python/wpt/test.py
@@ -644,17 +644,17 @@ def setUpModule():
         suppress_force_push=True,
     )
 
-    def setup_mock_repo(repo_name, local_repo):
+    def setup_mock_repo(repo_name, local_repo, default_branch: str):
         subprocess.check_output(
             ["cp", "-R", "-p", os.path.join(TESTS_DIR, repo_name), local_repo.path])
-        local_repo.run("init", "-b", "main")
+        local_repo.run("init", "-b", default_branch)
         local_repo.run("add", ".")
         local_repo.run("commit", "-a", "-m", "Initial commit")
 
     logging.info("=" * 80)
     logging.info("Setting up mock repositories")
-    setup_mock_repo("servo-mock", SYNC.local_servo_repo)
-    setup_mock_repo("wpt-mock", SYNC.local_wpt_repo)
+    setup_mock_repo("servo-mock", SYNC.local_servo_repo, "main")
+    setup_mock_repo("wpt-mock", SYNC.local_wpt_repo, "master")
     logging.info("=" * 80)