Hide Navigation Menu

I’m trying to create a landing page. I understand that there are plugins that one can buy. 9and free versions) I also use a plugin called WP popup and once potential clients sign up for my email list, my website directs to a thank you page on my website. I want to hide my websites navigation tabs for this thank you page. How can I do this? I have firebug, but it’s hard for me to correspond the html with the style in order to edit. Any videos on firebug that shows you how to match the html highlighted on the left of firebug with the style on the right? Anyway’s, here is the site that i want to remove the navigation tabbs from.

https://www.honestarttsentertainment.com/thankyou/