You've got me confused!
'Webmail' is a generic term referring to to web-based internet access to any email service. You could be using webmail from your ISP's email service, or from any of hundreds of other services. Without knowing which one you're referring to, it's impossible to give a precise answer.
However you simply need to go to the log-in page for the relevant service, and enter your account details. (If you don't know how to find the log-in page, tell us which email service you're using. i.e. what comes after the @ sign in your email address).
If you want to bookmark that log-in page for easy access in the future, then (with that page displayed) go to 'Bookmarks > Bookmark this page' (or simply press Ctrl and D together).
Alternatively, if you want your email log-in page to appear every time you start Firefox, then (with that page displayed) go to Tools > Options and ensure that the 'General' tab is selected. Set 'When Firefox starts' to 'Show my home page' and then click on 'Use Current Page' and 'OK'.