Make sure you are not blocking content.

Start Firefox in Safe Mode {web link}

A small dialog should appear. Click Start In Safe Mode (not Refresh). Is the problem still there?

https://support.mozilla.org/en-US/kb/enhanced-tracking-protection-firefox-desktop

This content is drawn on canvas.
Are there possibly canvas related messages in the Web Console?

• "3-bar" menu button or Tools -> Web Developer
• https://developer.mozilla.org/en-US/Tools/Web_Console

I see this issue when I enable "Resist Fingerprinting".

Are you possibly using Resist Fingerprinting?

• https://support.mozilla.org/en-US/kb/firefox-protection-against-fingerprinting

You can check this pref on the about:config page to see if Resist Fingerprinting is enabled and if this pref is user set (bold) reset to false via the right-click context menu.

• privacy.resistFingerprinting = false

If you want to keep using "Resist Fingerprinting" then set this pref to false on the about:config page and possibly reload the page.

• privacy.resistFingerprinting.autoDeclineNoUserInputCanvasPrompts = false

You can open the about:config page via the location/address bar. You can accept the warning and click "I accept the risk!" to continue.

• https://support.mozilla.org/en-US/kb/about-config-editor-firefox

See also "Tools -> Page Info -> Permissions -> Extract Canvas Data".