Update docstrings to specify the type of path needed to install firefox addons. (#4881)

This change helps the user give the correct full path to the addon they would like to install for firefox. Marionette is expecting a full path. Fixes #4858

Author: b4handjr

py/selenium/webdriver/firefox/firefox_profile.py

@@ -243,7 +243,7 @@ def _install_extension(self, addon, unpack=True):
         """
             Installs addon from a filepath, url
             or directory of addons in the profile.
-            - path: url, path to .xpi, or directory of addons
+            - path: url, absolute path to .xpi, or directory of addons
             - unpack: whether to unpack unless specified otherwise in the install.rdf
         """
         if addon == WEBDRIVER_EXT:

py/selenium/webdriver/firefox/webdriver.py

@@ -237,8 +237,10 @@ def install_addon(self, path, temporary=None):
         Returns identifier of installed addon. This identifier can later
         be used to uninstall addon.
 
+        :param path: Absolute path to the addon that will be installed.
+
         :Usage:
-            driver.install_addon('firebug.xpi')
+            driver.install_addon('/path/to/firebug.xpi')
         """
         payload = {"path": path}
         if temporary is not None: