THIS SCRIPT NO LONGER WORKS! Twitter has rolled out a fix for the web client hack. (Original text: Force enable cramming (280 character tweets) on Twitter. Use TamperMonkey. NOTE: Stops working when you switch pages, refresh to fix.)

Readme.md

As of 7 November 2017 everyone has access to 280 characters in supported clients, so you no longer need this script!

Twitter Cramming.user.js

// ==UserScript==
// @name Twitter Cramming
// @description Force enable cramming (280 character tweets) on Twitter
// @author Prof. 9
// @version 0.1
// @match https://twitter.com/*
// @run-at document-idle
// @namespace prof9.twittercramming
// ==/UserScript==

var initDataInput = document.getElementById('init-data');
var initData = JSON.parse(initDataInput.value);
initData.deciders.cramming_feature_enabled = true;
initData.deciders.cramming_ui_enabled = true;
initDataInput.value = JSON.stringify(initData);

// This is free and unencumbered software released into the public domain.
// 
// Anyone is free to copy, modify, publish, use, compile, sell, or
// distribute this software, either in source code form or as a compiled
// binary, for any purpose, commercial or non-commercial, and by any
// means.
// 
// In jurisdictions that recognize copyright laws, the author or authors
// of this software dedicate any and all copyright interest in the
// software to the public domain. We make this dedication for the benefit
// of the public at large and to the detriment of our heirs and
// successors. We intend this dedication to be an overt act of
// relinquishment in perpetuity of all present and future rights to this
// software under copyright law.
// 
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
// EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
// MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
// IN NO EVENT SHALL THE AUTHORS BE LIABLE FOR ANY CLAIM, DAMAGES OR
// OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE,
// ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
// OTHER DEALINGS IN THE SOFTWARE.
// 
// For more information, please refer to <http://unlicense.org>

TD.services.TwitterClient.prototype.makeTwitterCall=function(b,e,f,g,c,d,h){c=c||function(){};d=d||function(){};b=this.request(b,{method:f,params:Object.assign(e,{weighted_character_count:!0}),processor:g,feedType:h});return b.addCallbacks(function(a){c(a.data)},function(a){d(a.req,"",a.msg,a.req.errors)}),b};
twttrTxt=Object.assign({},twttr.txt,{isInvalidTweet:function(){return!1},getTweetLength:function(){return twttr.txt.getTweetLength.apply(this,arguments)-140}});