The best Email Client alternative to Mozilla Firefox is Vivaldi, which is free. If that doesn't suit you, our users have ranked more than 100 alternatives to Mozilla Firefox and three of them are Email Clients so hopefully you can find a suitable replacement. Other interesting Email Client alternatives to Mozilla Firefox are SeaMonkey and Iceape-UXP.
Flexible browser featuring deep customization, integrated ad and tracker blocking, Chromium compatibility, powerful tab management tools, note sync, screenshot utility, custom web panels, theme and toolbar editing, plus support for Android, iOS, desktop, and privacy protections.
Web-browser, advanced e-mail, newsgroup and feed client, IRC chat, and HTML editing in one application. Itis built on the open source Mozilla Gecko engine, the same code which underlies Thunderbird and is the base for the Firefox browser.
Historically, Iceape was a re-branding of SeaMonkey by the Debian project. However, Debian discontinued Iceape in December 2013 citing security concerns and lack of a maintainer.
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Vivaldi is simply the next stop for Firefox users, while it is freeware, it is technically closed and open-source. Some of the browser's components are closed while some are open and the source codes are available on Vivaldi's website.