doc: "Troubleshooting" section + minor changes

docs/codesnippets/92_import_firefox_session.py

@@ -0,0 +1,23 @@
+from glob import glob
+from os.path import expanduser
+from sqlite3 import connect
+
+from instaloader import ConnectionException, Instaloader
+
+# FIREFOXCOOKIEFILE = "/home/alex/.mozilla/firefox/l96w6b90.default/cookies.sqlite"
+FIREFOXCOOKIEFILE = glob(expanduser("~/.mozilla/firefox/*.default/cookies.sqlite"))[0]
+
+instaloader = Instaloader(max_connection_attempts=1)
+instaloader.context._session.cookies.update(connect(FIREFOXCOOKIEFILE)
+                                            .execute("SELECT name, value FROM moz_cookies "
+                                                     "WHERE baseDomain='instagram.com'"))
+
+try:
+    username = instaloader.test_login()
+    if not username:
+        raise ConnectionException()
+except ConnectionException:
+    raise SystemExit("Cookie import failed. Are you logged in successfully in Firefox?")
+
+instaloader.context.username = username
+instaloader.save_session_to_file()