Make sure you have the latest FFmpeg and libavcodec packages installed.

You can check the Web Console for media related messages.

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

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You can reload web page(s) and bypass the cache to refresh possibly outdated or corrupted files.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 the Cookies for websites that cause problems via the "3-bar" Firefox menu button (Options/Preferences).

"Remove the Cookies" for websites that cause problems:

• Options/Preferences -> Privacy & Security
  Cookies and Site Data: "Manage Data"

"Clear the Cache":

• Options/Preferences -> Privacy & Security
  Cookies and Site Data -> Clear Data -> Cached Web Content: Clear

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

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

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

• https://support.mozilla.org/en-US/kb/enhanced-tracking-protection-firefox-desktop
• https://support.mozilla.org/en-US/kb/trackers-and-scripts-firefox-blocks-enhanced-track

On support advice above went to developer options in main menu. Confusing error messages displayed. Some say security is causing blocks, some say codecs missing. How do I load the missing codecs. Which security settings need altering? Content Security Policy: Ignoring “https://www.instagram.com” within script-src: ‘strict-dynamic’ specified Content Security Policy: Ignoring “https://www.ons.gov.uk” within script-src: ‘strict-dynamic’ specified Content Security Policy: Ignoring “https://gn-web-assets.api.bbc.com” within script-src: ‘strict-dynamic’ specified Content Security Policy: Ignoring “https://www.google-analytics.com” within script-src: ‘strict-dynamic’ specified Content Security Policy: Ignoring “https://static.files.bbci.co.uk” within script-src: ‘strict-dynamic’ specified HTTP "Content-Type" of "video/mp4" is not supported. Load of media resource https://emp.bbci.co.uk/emp/media/blankv2.mp4 failed. iframe.html XML Parsing Error: syntax error Location: https://emp.bbc.co.uk/emp/SMPj/2.36.2/iframe.html Line Number 1, Column 1: iframe.html:1:1 The video on this page can't be played. Your system may not have the required video codecs for: video/mp4;codecs="avc3.4D4015";width="512";height="288", video/mp4;codecs="avc3.4D401F";width="704";height="396", video/mp4;codecs="avc3.64001F";width="960";height="540", video/mp4;codecs="avc3.64001F";width="960";height="540", video/mp4;codecs="avc3.640020";width="1280";height="720", video/mp4;codecs="avc3.42C015", audio/mp4;codecs="mp4a.40.5" iframe.html HTTP "Content-Type" of "application/vnd.apple.mpegurl" is not supported. Load of media resource https://mm.bidi.bbc.co.uk/vod-hls-uk/usp/auth/vod/piff_abr_full_hd/84cd5c-p08d5ql6/vf_p08d5ql6_165faf91-d8a6-4295-8304-11e79b539756.ism/pc_hd_abr_v2_hls_master.m3u8?at=kjK3BLDEddd8c1869e70b1f42cdae8faf574bc57602159fd5b30b1aab4f40 failed. iframe.html The video on this page can't be played. Your system may not have the required video codecs for: video/mp4;codecs="avc3.4D4015";width="512";height="288", video/mp4;codecs="avc3.4D401F";width="704";height="396", video/mp4;codecs="avc3.64001F";width="960";height="540", video/mp4;codecs="avc3.64001F";width="960";height="540", video/mp4;codecs="avc3.640020";width="1280";height="720", video/mp4;codecs="avc3.42C015", audio/mp4;codecs="mp4a.40.5" iframe.html