Location of userContent.css in Linux and Windows 7

Windows 7

C:\Users\{username}\AppData\Roaming\Mozilla\Firefox\Profiles\{profile-directory}\chrome

E.g.

C:\Users\arun\AppData\Roaming\Mozilla\Firefox\Profiles\4s7v2bl8.default\chrome

Linux

/home/{username}/.mozilla/firefox/{profile-directory}/chrome

E.g.

/home/arun/.mozilla/firefox/4s7v2bl8.default/chrome

View Gmail conversations in fixed width font using Firefox

Add the following lines to "$HOME/.mozilla/firefox/{profile-directory}/chrome/userContent.css" to view Gmail emails in fixed width font using Firefox.

@-moz-document domain(mail.google.com)
{
    /* GMail messages and textarea should use fixed-width font */
    .gs .ii, textarea.Ak {
        font-family: MonoSpace !important;
        font-size: 10pt !important;
    }
}

That's it. Enjoy.
Edit: 13 Feb 2016 (Originally published on 09 Oct 2011)
This still works on Firefox 44 on Fedora 23. The "chrome" directory was missing in profiles directory. Just create the directory and put "userContent.css" file inside it.