install_addon() didn't take into account dir paths with trailing slashes

If install_addon() was called with a path argument pointing to
to an extension directory and the path argument value itself ended
with a trailing slash, install_addon() would eat the first
character of each file it would be adding to the synthetized xpi file.

Fixes #13685

Author: jkbzh

py/selenium/webdriver/firefox/webdriver.py

@@ -125,7 +125,10 @@ def install_addon(self, path, temporary=False) -> str:
 
         if os.path.isdir(path):
             fp = BytesIO()
-            path_root = len(path) + 1  # account for trailing slash
+            # filter all trailing slash found in path
+            path = path.rstrip(os.sep)
+            # account for trailing slash that will be added by os.walk()
+            path_root = len(path) + 1
             with zipfile.ZipFile(fp, "w", zipfile.ZIP_DEFLATED) as zipped:
                 for base, _, files in os.walk(path):
                     for fyle in files: