correcting one instance of self.platform usage(#8956)

Yesterday I opened a pull request: #8953 however, I forgot to replace `platform` with `self.platform`

Author: Machinexa2

py/selenium/webdriver/firefox/firefox_binary.py

@@ -85,7 +85,7 @@ def kill(self):
     def _start_from_profile_path(self, path):
         self._firefox_env["XRE_PROFILE_PATH"] = path
 
-        if platform == 'linux':
+        if self.platform == 'linux':
             self._modify_link_library_path()
         command = [self._start_cmd, "-foreground"]
         if self.command_line: