Does this only happen with dark mode on YouTube?

It is possible that you have black text on black background.

Does any text show if you Ctrl+A to select all the text?

It also happens on light mode, and ctrl+a does nothing, most of the page appears to not be loaded. I can't scroll down either

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You can reload web page(s) and bypass the cache to refresh possibly outdated or corrupted files.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

"Remove the Cookies" from 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 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

I've tried all of the above and problem is unfortunately still there.