Firefox - border around form goes full page width

You have some invalid markup in that page. Every browser interprets invalid markup in a different way, and it's likely some browsers look OK and some look messed up.

I see some extra markup being added to the Gravity Forms shortcode output. There are extra closing </ p > tags around the closing form tag and div. Here is a snippet of the source:

[html]
<input type='hidden' name='gform_field_values' value='' /></div>
</p></form>
</p></div>

Those closing < p > tags should not be present on line 2 and line 3. They are not output by Gravity Forms.

Looks like Modernize is a Themeforest theme. They typically include the [raw] shortcode in all their themes and that can cause this extra markup to be added to the gravityforms shortcode output. Additional reading: http://www.gravityhelp.com/forums/tags/raw

The quick fix is to wrap the gravityforms shortcode in the [raw] shortcode. That prevents the Gravity Forms output from being modified. That is not a fix for the problem, just a workaround.

Please let us know if you need more help.