Hi Nisaba, there are two different feature: Email Link and Share. For Email Link, your can let Firefox know which site to use for compose. This article has the steps: [[Change the program used to open email links]]. Firefox doesn't store your account details, so answering your question depends on whether you changed your username but you are still using the same service (for example, two different @yahoo''.''com addresses) or if you are changing mail sites completely -- for example, using Outlook instead of Gmail. How you set up a new site varies, so it would be useful to learn more about how you compose email messages now to see whether we can answer that or whether we would need help from your new mail site. The Share feature, on the other hand, calls up a Windows 10 panel, so those settings are outside of Firefox.