You can check the Web Console for error messages.

• https://firefox-source-docs.mozilla.org/devtools-user/web_console/

Clear the Cache and remove the Cookies for websites that cause problems via the "3-bar" Firefox menu button (Settings).

"Remove the Cookies" for websites that cause problems:

• Settings -> Privacy & Security
  Cookies and Site Data: "Manage Data"

"Clear the Cache":

• Settings -> Privacy & Security
  Cookies and Site Data -> Clear Data -> [X] Cached Web Content -> Clear

• https://support.mozilla.org/en-US/kb/clear-cookies-and-site-data-firefox
• https://support.mozilla.org/en-US/kb/how-clear-firefox-cache

You can remove all data stored in Firefox for a specific domain via "Forget About This Site" in the right-click context menu of an history entry ("History -> Show All History" or "View -> Sidebar -> History").

Using "Forget About This Site" will remove all data stored in Firefox for this domain like history and cookies and passwords and exceptions and cache, so be cautious. If you have a password or other data for that domain that you do not want to lose, make sure to backup this data or make a note.

You can't recover from this 'forget' unless you have a backup of involved files.

If you revisit a 'forgotten' website, data for that website will be saved once again.

• https://support.mozilla.org/en-US/kb/delete-browsing-search-download-history-firefox

The forget this site seems to have resolved it, thank you. There have been times that the site might not work for me today but then I try it later or tomorrow and it loads (but that is only a FF issue as Edge still works). I usually try clear cache first thing, sorry I forgot to mention. I use Shift Control Delete usually to delete cache. The site appears to use quirky mode. I sometimes get the same issue with rightstufanime.com as well, will try the forget site option there too to see if it resolved.