8GB of ram... Just curious to know which mac are you on? You fail to provide detail on what exactly you have open when this issue occurs. When I visit the site, there is a bump in cpu usage, but it appears to be a lot going on with this site. You can see the windows and tabs I have open, but no issue. see screenshot

You can consider uBlock Origin as a replacement for your current Adblocker extension to see if that works better (lower memory footprint,less issues).

• https://addons.mozilla.org/firefox/addon/ublock-origin/

Start Firefox in Troubleshoot Mode to check if one of the extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) or if hardware acceleration or if userChrome.css/userContent.css is causing the problem.

• switch to the Default System theme: "3-bar" menu button or Tools -> Add-ons -> Themes
• do NOT click the "Refresh Firefox" button on the Troubleshoot Mode start window

• https://support.mozilla.org/en-US/kb/diagnose-firefox-issues-using-troubleshoot-mode
• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems

Does it still happen in Troubleshoot Mode?

Problem does not occur in Troubleshoot Mode. (Thanks for suggestion, I was unaware of TS mode). Based on the number of adverts that appeared with TS turned on, I suspect the issue is linked to my use of Adblock Plus. Per, cor-el's suggestion, I will I try Ublock Origin for a smaller memory footprint. I will play around with this for a few days and report back.

Problem has not re-occurred since replacing Adblock Plus with Ublock Origin.