Userscripts.org

...so I fixed it for you.
Change line 12 from:
document.location.href = "https://twitter.com" + document.location.pathname;
To:
document.location.href = "https://twitter.com" + document.location.pathname + document.location.search + document.location.hash;

document.location.search is the ?lang=en part in http://twitter.com/?lang=en
document.location.hash is the #!/firefox part in http://twitter.com/#!/firefox

In the script as it is now, http://twitter.com/?lang=en and http://twitter.com/#!/firefox both redirect to https://twitter.com/.

Please update the script with this change. :)