Do you think you might have blocked the site from customizing keyboard shortcuts? When you modify a site permission from the default, when you are on the page, Firefox shows an extra icon next to the lock that looks like two lines with circles on different ends. If you click that, it will show the customized permission(s).

If you are referring to the external controls that pop up outside of the window, there is a general master switch in about:config. Unless you recall going into about:config, this probably is okay, but just in case:

(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button accepting the risk.

More info on about:config: Configuration Editor for Firefox. The moderators would like us to remind you that changes made through this back door aren't fully supported and aren't guaranteed to continue working in the future.

(2) In the search box in the page, type or paste media.hardwaremediakeys.enabled and pause while the list is filtered

(3) If the preference is bolded and has a value of false, double-click it (or click the Toggle button or Reset button at the right end of the row) to restore the default value of true

Hi. I've looked for the customised permissions logo as you mentioned but there's only the shield logo next to the lock logo for the webpage.

I've also checked the hardware media controls in Firefox settings and they are enabled. I used about:config and the "media.hardwaremediakeys.enabled" value is true. I've also went to Settings > General > Browsing and "Control media via keyboard, headset, or virtual interface" is enabled.

The settings box that appeared was a small black box on the bottom right which mentioned what key you need to hit to pause (spacebar), forward 10 secs (right arrow), rewind 10 secs (left arrow) etc. and a scroll bar to either enable or disable these keyboard shortcuts. Could this be something specific for Lenovo? This is a Lenovo IdeaPad 5 laptop

If the popup was outside the page and had controls, it does sound like it would have been from the system. If it was part of the page, maybe it was a setting saved in a cookie. You could test the page in a private window, since that will not share cookies or local storage data with your regular windows. If it works normally there, you could clear the site's cookies selectively in the regular window using Shift+F9 (better if the site stores progress information locally), or in bulk by clicking the lock icon and then "Clear cookies and site data..."

That worked, thanks so much! It must've been a setting saved in a cookie as you mentioned because the playback shortcut keys started working again once I cleared the history & cookies on Firefox