Make sure you are not blocking content.

https://support.mozilla.org/en-US/kb/enhanced-tracking-protection-firefox-desktop also see https://blog.mozilla.org/security/2021/03/23/introducing-smartblock/

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

Diagnose Firefox issues using Troubleshoot(Safe) Mode {web link}

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

Many site issues can be caused by corrupt cookies or cache.

Warning ! ! This will log you out of sites you're logged in to. You may also lose any settings for that website.

• Delete cookies to remove the information websites have stored on your computer {web link}

• How to clear the Firefox cache {web link}

As I suspected, the cure will cause me a big headache. Twitter will open as usual in safe mode which means I have to refresh Firefox and loose all the extensions and addons I like. This problem happened simply after closing down as usual and re starting after a few hours. Nothing new was added, no changes made. I'm nearly eighty years old and find keeping up with these things quite difficult. I use Twitter a lot and may have to change to another browser which will be a shame as I've used Firefox trouble free for years, but there's obviously a glitch in the works and I don't know if I can trust it any more. If there's no simple solution It's going to be bye to Firefox. Thanks for the help anyway.

Ghostery gives me a blank twitter page.

Thank you very much. Disabled Ghosterly and the problem was solved.

Good to hear the problem was solved. Safe Surfing.

Thanks to all for the advice