You may have accidentally zoomed webpage(s). Reset the page zoom on pages that cause problems.

• View -> Zoom -> Reset (Ctrl+0/Command+0 (zero))

• https://support.mozilla.org/en-US/kb/font-size-and-zoom-increase-size-of-web-pages

thank you for such a quick response, but no that has not fixed the issue unfortunately.

A full browser screenshot would be more helpful.

Clear the Cache and remove the Cookies for websites that cause problems via the "3-bar" Firefox menu button (Settings).

"Remove the Cookies" for websites that cause problems:

• Settings -> Privacy & Security
  Cookies and Site Data: "Manage Data"

"Clear the Cache":

• Settings -> Privacy & Security
  Cookies and Site Data -> Clear Data -> [X] Cached Web Content -> Clear

• https://support.mozilla.org/en-US/kb/clear-cookies-and-site-data-firefox
• https://support.mozilla.org/en-US/kb/how-clear-firefox-cache

You can remove all data stored in Firefox for a specific domain via "Forget About This Site" in the right-click context menu of an history entry ("History -> Show All History" or "View -> Sidebar -> History").

Using "Forget About This Site" will remove all data stored in Firefox for this domain like history and cookies and passwords and exceptions and cache, so be cautious. If you have a password or other data for that domain that you do not want to lose then make sure to backup this data or make a note.

You can't recover from this 'forget' unless you have a backup of involved files.

If you revisit a 'forgotten' website then data for that website will be saved once again.

• https://support.mozilla.org/en-US/kb/delete-browsing-search-download-history-firefox

A full browser screenshot would be more helpful.

I tend to multitask, so I usually have two windows up, which is why I prefer to use standard mode on YouTube. But I have included full desktop screenshots. First is standard mode when the browser window isn't full screen. Second is theater mode when the browser isn't full screen. Third is standard mode when the browser is full screen. It seems to only be an issue when in standard mode and the browser isn't full screen.

Unfortunately none of these have fixed the issue either. But again thank you for the quick response, and for posting the helpful links to talk me through the processes!

Thanks for the pics. Did you ever try Safe Mode?

Thanks for the pics. Did you ever try Safe Mode?

How do I enable safe mode?

Safe Mode. https://support.mozilla.org/en-US/kb/diagnose-firefox-issues-using-troubleshoot-mode#w_how-to-start-firefox-in-4troubleshoot-modesf5safe-modesf

Still have the issue in Safe Mode unfortunately.

Came back to youtube ~60 hours after the issue started and it's no longer an issue!

Glad to hear, mark it as resolved and have a good day.