That is a possibility. Not sure if we got all your info as do not see any add-ons/Extensions listed. So no idea if running a adblocker. If not could check out Ghostery https://addons.mozilla.org/en-US/firefox/addon/ghostery/?src=search and Ublock Origin https://addons.mozilla.org/en-US/firefox/addon/ublock-origin/?src=search as possible help to see if the 1 site loads faster. also Fat-Free-Fox https://addons.mozilla.org/en-US/firefox/addon/fat-free-firefox/ Please let us know if this solved your issue or if need further assistance.

I see this happen as well in Firefox. A look in the Network Monitor shows a lot of XHR requests that may account for this behavior. *https://developer.mozilla.org/Tools/Network_Monitor