diff --git a/py/selenium/webdriver/firefox/options.py b/py/selenium/webdriver/firefox/options.py
index <HASH>..<HASH> 100644
--- a/py/selenium/webdriver/firefox/options.py
+++ b/py/selenium/webdriver/firefox/options.py
@@ -153,7 +153,7 @@ class Options(ArgOptions):
         if len(self._preferences) > 0:
             opts["prefs"] = self._preferences
         if self._proxy is not None:
-            self._proxy.add_to_capabilities(opts)
+            self._proxy.add_to_capabilities(caps)
         if self._profile is not None:
             opts["profile"] = self._profile.encoded
         if len(self._arguments) > 0:
[py] Pass capabilities rather than options to the proxy object (#<I>)

The method add_to_capabilities from the Proxy class takes capabilities

(self._caps) as argument instead of "opts"