Download link: https://addons.mozilla.org/thunderbird/addon/always-default-client/
Makes Firefox/Thunderbird as the default client on every startup.
You can control the behavior by following preferences:
// true => set default for all users on the computer (*require administrator permission)
user_pref("extensions.alwaysdefaultclient@clear-code.com.allUsers", false);
// true => use Firefox for http, https, .xhtml, etc.
// false => use Firefox for http only
user_pref("extensions.alwaysdefaultclient@clear-code.com.browser.allTypes", false);
// 1 = Mail
// 2 = News
// 4 = Feed
// (1 + 2 + 4 = 7 = All)
user_pref("extensions.alwaysdefaultclient@clear-code.com.mail.types", 1);
This is mainly designed for corporate-use.