Do you mean web-based email? If so, which web-based email provider, and what happens when you try? At what stage does checking email fail, and what error messages do you see?
If you mean email client software like Thunderbird, Outlook, Windows Live Mail, etc, then that does not need Firefox.
So, tell us more.
My operating system is Windows 7. I was able to check mail using Firefox but since yesterday I just could not. Please help. Thanks. Angeline