Gmail – Export Messages to Text or HTML Files

You could add your Gmail account as a new IMAP account to Thunderbird. From there you can just drag them from your old account folder to the Gmail/[All Mail] folder.

See http://email.about.com/od/gmailtips/qt/How_to_Import_Mail_from_Mozilla_Thunderbird_in_Gmail.htm for steps.

Update 2019-07-15:

The gmail-fixed-font extension is now hosted on github: https://github.com/jparise/gmail-fixed-font

The instructions are provided in the repo README, but to summarise (assuming you are using Chrome):

1. Download the script
2. Open a new tab and navigate to chrome://extensions/
3. Enable Developer Mode
4. Drag and drop the script onto the window and install
5. (optional) you may want to allow the extension only for the gmail domain by clicking Details > Site Access: On specific sites and enter https://mail.google.com in the dialogue box and save

Ok, you have to try Gmail Fixed Font userscript

// ==UserScript==
// @name           Gmail Fixed Font
// @namespace      http://www.indelible.org/
// @description    Fixed-font message bodies for Gmail
// @author         Jon Parise, James Tunnicliffe
// @version        1.3
// @include        http://mail.google.com/*
// @include        https://mail.google.com/*
// @include        http://*.mail.google.com/*
// @include        https://*.mail.google.com/*
// @grant          GM_addStyle
// ==/UserScript==

// Plain-text Message Body
var css = ".ii, .Ak { font: medium monospace !important; }";
// Compose Interface
css += ".editable { font: medium monospace !important; }";

if (typeof GM_addStyle != "undefined") {
    GM_addStyle(css);
} else if (typeof addStyle != "undefined") {
    addStyle(css);
} else {
    var heads = document.getElementsByTagName("head");
    if (heads.length > 0) {
        var node = document.createElement("style");
        node.type = "text/css";
        node.appendChild(document.createTextNode(css));
        heads[0].appendChild(node);
    }
}