The user agent issue might be caused by changing this setting in the about:config preferences editor:

privacy.resistFingerprinting => true

I don't know whether that would have any effect on keyboard shortcuts. That doesn't sound like a function that would need to be suppressed.

When I tested five keys in a private window (so, logged out), these worked for me: j, k, l, <, >

Since these are plain keys (no Ctrl, etc.), I don't think Firefox's keyboard shortcuts permission would be relevant. However, you could give YouTube specific permission to override keyboard shortcuts to see whether that makes any difference:

While you are on the site, you can call up the Permissions panel of the Page Info dialog using any of these:

• right-click a blank area of the page and choose View Page Info > Permissions
• (menu bar) Tools menu > Page Info > Permissions

Scroll down to "Override Keyboard Shortcuts" and uncheck the "Use default" box, and then select the "Allow" permission.

(Sorry, the parts in quotation marks are in a different language on yours!)