diff --git a/CHANGELOG.md b/CHANGELOG.md
index 2b8826b6e..eb89f3197 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,11 @@
 # Release History
 
+## Unreleased: mitmproxy next
+
+* Don't negotiate HTTP/2 on the outer TLS connection in Secure Web Proxy Mode.
+  This fixes compatibility with Firefox.
+
+
 ## 16 July 2021: mitmproxy 7.0
 
 ### New Proxy Core (@mhils, [blog post](https://www.mitmproxy.org/posts/releases/mitmproxy7/))
diff --git a/mitmproxy/addons/tlsconfig.py b/mitmproxy/addons/tlsconfig.py
index f514e5e0e..1f210d721 100644
--- a/mitmproxy/addons/tlsconfig.py
+++ b/mitmproxy/addons/tlsconfig.py
@@ -8,10 +8,11 @@ from mitmproxy import certs, ctx, exceptions, connection
 from mitmproxy.net import tls as net_tls
 from mitmproxy.options import CONF_BASENAME
 from mitmproxy.proxy import context
-from mitmproxy.proxy.layers import tls
+from mitmproxy.proxy.layers import tls, modes
 
 # We manually need to specify this, otherwise OpenSSL may select a non-HTTP2 cipher by default.
 # https://ssl-config.mozilla.org/#config=old
+
 DEFAULT_CIPHERS = (
     'ECDHE-ECDSA-AES128-GCM-SHA256', 'ECDHE-RSA-AES128-GCM-SHA256', 'ECDHE-ECDSA-AES256-GCM-SHA384',
     'ECDHE-RSA-AES256-GCM-SHA384', 'ECDHE-ECDSA-CHACHA20-POLY1305', 'ECDHE-RSA-CHACHA20-POLY1305',
@@ -149,8 +150,17 @@ class TlsConfig:
             dhparams=self.certstore.dhparams,
         )
         tls_start.ssl_conn = SSL.Connection(ssl_ctx)
+
+        # Force HTTP/1 for secure web proxies, we currently don't support CONNECT over HTTP/2.
+        # There is a proof-of-concept branch at https://github.com/mhils/mitmproxy/tree/http2-proxy,
+        # but the complexity outweighs the benefits for now.
+        if len(tls_start.context.layers) == 2 and isinstance(tls_start.context.layers[0], modes.HttpProxy):
+            client_alpn: Optional[bytes] = b"http/1.1"
+        else:
+            client_alpn = client.alpn
+
         tls_start.ssl_conn.set_app_data(AppData(
-            client_alpn=client.alpn,
+            client_alpn=client_alpn,
             server_alpn=server.alpn,
             http2=ctx.options.http2,
         ))
diff --git a/mitmproxy/proxy/layers/tls.py b/mitmproxy/proxy/layers/tls.py
index c9a2ff2b3..bb2cfd492 100644
--- a/mitmproxy/proxy/layers/tls.py
+++ b/mitmproxy/proxy/layers/tls.py
@@ -346,6 +346,23 @@ class ClientTLSLayer(_TLSLayer):
     client_hello_parsed: bool = False
 
     def __init__(self, context: context.Context):
+        if context.client.tls:
+            # In the case of TLS-over-TLS, we already have client TLS. As the outer TLS connection between client
+            # and proxy isn't that interesting to us, we just unset the attributes here and keep the inner TLS
+            # session's attributes.
+            # Alternatively we could create a new Client instance,
+            # but for now we keep it simple. There is a proof-of-concept at
+            # https://github.com/mitmproxy/mitmproxy/commit/9b6e2a716888b7787514733b76a5936afa485352.
+            context.client.alpn = None
+            context.client.cipher = None
+            context.client.sni = None
+            context.client.timestamp_tls_setup = None
+            context.client.tls_version = None
+            context.client.certificate_list = []
+            context.client.mitmcert = None
+            context.client.alpn_offers = []
+            context.client.cipher_list = []
+
         super().__init__(context, context.client)
         self.server_tls_available = isinstance(self.context.layers[-2], ServerTLSLayer)
         self.recv_buffer = bytearray()