If you want to check your gmail in firefox then uninstall Outlook Express/Microsoft Outlook and simply browse to
http://gmail.google.co m or
http://gmail.com/ to check your gmail.
If you mean that you want to check your gmail using Outlook express then do the following:
In Outlook Express, set both POP3 and SMTP servers to 127.0.0.1, (NOT ***.gmail.com). Assign the POP3 port number as 995, and the SMTP port nunber to 587 or 465. DO NOT check "This server requires a secure connection (SSL)" on the advanced tab. If you have these checked and uncheck them, it will reset your SMTP to 25 and your POP to 110, so remember to type in the correct port numbers after you uncheck the SSL boxes.
On the Servers tab, DO NOT check "Log on using secure password authentication." DO check "My Server requires authentication" and in the "settings" button for that panel make sure that the radio button for "Use same settings as my incoming mail server" is checked.
Make sure you click "apply" on each window before backing out. Also, make sure that you double check the settings.
These are all of the bare settings, without an explanation:
Servers tab
POP3 server name 127.0.0.1
SMTP server name 127.0.0.1
You MUST enable authentication (logging into) your Outgoing server
Advanced tab
POP3 server port 995
SMTP server port 465
DO NOT check "This server requires a secure connection (SSL)" on the advanced tab for either POP3 or SMTP