It could be that your bookmarks toolbar has disappeared from Firefox.
With Firefox open, right-click near to the top of the screen. If there's not a tick next to 'Bookmarks Toolbar', click to put one there.
That might be all you need to do. If not, (with the bookmarks toolbar in place) go here:
https://signin1.bt.com/login/emailloginform?s_cid=con_FURL_email
Click on where it says 'British Telecommunications plc' (just to the left of the address bar) and, keeping your mouse button held down, drag it onto the bookmarks toolbar
You should then have a shortcut labelled 'Email Login Page'. If you'd rather have a different name (such as 'BT Mail'), right-click on the shortcut, choose 'Properties, change the name and click 'Save'.