Currently part of toolkit/content/charsetOverlay.js is duplicated in browser.js. I'm not sure why, but ideally we would be able to share this code.

Worth noting that before toolkit even forked them, WeirdAl was on the hook to break up the xpfe XUL and JS files into separate per-app-portion files, in bug 120970. I've never really looked at how possible it would be to generalize all the MailThisFunction and composerThatMenus, but I've always thought it rather odd that toolkit had a pair of mail/composer/browser files that only view source and Thunderbird use.