I am using an ad blocker, but don't get a complaint from that webpage. But I do get a message about cookies at the top, which isn't persistent. It goes away when I scroll down the page and doesn't reappear when I scroll back to the top or when I reload that page.

You can try these steps in case of issues with web pages:

You can reload web page(s) and bypass the cache to refresh possibly outdated or corrupted files.

• Hold down the Shift key and left-click the Reload button
• Press "Ctrl + F5" or press "Ctrl + Shift + R" (Windows,Linux)
• Press "Command + Shift + R" (Mac)

Clear the cache and remove cookies from websites that cause problems.

"Clear the Cache":

• Firefox/Tools > Options > Advanced > Network > Cached Web Content: "Clear Now"

"Remove Cookies" from websites that cause problems:

• Firefox/Tools > Options > Privacy > "Use custom settings for history" > Cookies: "Show Cookies"

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

• Switch to the DEFAULT theme: "3-bar" menu button/Tools > Add-ons > Appearance
• Do NOT click the Reset button on the Safe Mode start window

• https://support.mozilla.org/kb/Safe+Mode
• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes