There are a lot of pre-assigned keyboard shortcuts that Firefox allows websites to intercept and/or override. If you want to deny a site the ability to override a built-in shortcut, you could try adjusting the permission as follows:

While you are on a page on the site, call up the Page Info dialog using one of these methods:

• Ctrl+i (for Mac, Command+i)
• (menu bar) Tools > Page Info
• click the lock icon in the address bar, on the drop-down click the > button, then at the bottom click "More Information"

When the dialog comes up, click the Permission icon at the top to show that panel.

Scroll down to "Override Keyboard Shortcuts" and uncheck the "Use default" box, and then select Block to bar overriding

(There's no Save button for this dialog, changes are saved in real time.)

Does that work? Does it break any other shortcuts you actually need?

Thank you for the quick response! That solved it!