You will need to install a user script manager extension to install this script.
Full subreddit menu displays all your subreddits in the "My subreddits" dropdown menu instead of the default list capped at 50 items. It does this by AJAX getting your subscriptions from reddit.com/subreddits once per session and on demand via the "Refresh" link in the bottom of the menu. SessionStorage is used to avoid additional HTTP requests on every page load. It also utilizes the screen width to display a whole lot more subreddits.