Rendering issue in Firefox 19 when replacing default document

[azappella]

Hi,

It seems that if you try to replace the default markdown document in app-model.js (l. 198-223):

onboarding = [
    '# Socrates now supports LaTeX Math!\n',
    'If you haven\'t used it before, Socrates is a real-time [markdown](http://daringfireball.net/projects/markdown/) editor in your browser. And **today we added support for beautiful math**, using [MathJax](http://mathjax.com/)!\n',

    'Let\'s derive the quadratic formula. Starting with:',
    '$$ ax^2 + bx + c = 0 $$\n',

    'We\'ll move around the constant terms and coefficients:',
    '$$ ax^2 + bx = -c $$',
    '$$ x^2 + \\frac{b}{a} x = -\\frac{c}{a} $$\n',

    'Then we add a new mystery constant so that things will simplify later:',
    '$$ x^2 + \\frac{b}{a} x + \\frac{b^2}{4a^2} = -\\frac{c}{a}  + \\frac{b^2}{4a^2} $$\n',

    'Then it\'s a simple matter of boiling this down to the final solution:',
    '$$ x^2 + \\frac{b}{a} x + \\frac{b^2}{4a^2} = \\frac{b^2 - 4ac}{4a^2} $$',
    '$$ \\left( x + \\frac{b}{2a} \\right)^2 = \\frac{b^2 - 4ac}{4a^2} $$',
    '$$ x + \\frac{b}{2a} = \\pm \\sqrt{ \\frac{b^2 - 4ac}{4a^2} } $$',
    '$$ x = - \\frac{b \\pm \\sqrt{ b^2 - 4ac } }{2a} $$\n',

    '$$ \\Box $$\n',

    'Keeping all of the LaTeX math syntax in your head is no easy feat, but you\'ll find the [Art of Problem Solving Wiki](http://www.artofproblemsolving.com/Wiki/index.php/LaTeX:Symbols) is an invaluable reference.\n',

    'Happy mathing!'
].join('\n');

by something such as:

onboarding = [
    '# Placeholder Title',
].join('\n');

There is a rendering issue in Firefox 19.0 (Windows 7). It works fine in Chrome, though.

It also seems that the rendering issue doesn't appear when some Latex is provided, such as:

onboarding = [
    '# Placeholder Title',
    '$$ ax^2 + bx + c = 0 $$\n',
].join('\n');