What purpose of those keyboard shortcuts is this about as they can have their own effect in a different use case ?

Like in the location bar it allows to cycle trough the search engines and the keyboard shortcuts article linked above use it to (un)mute the audio in the media player.

Here are the Firefox shortcuts or hotkeys by default on the Browser. https://support.mozilla.org/en-US/kb/keyboard-shortcuts-perform-firefox-tasks-quickly https://firefox-source-docs.mozilla.org/devtools-user/keyboard_shortcuts/index.html Any other keyboard combo is either 3rd party or userChrome.css edits or is O/S keyboard commands not used in Firefox.

Soo, it means I can add my own keyboard shortcuts throught userChrome.css? How to do this?

What purpose of those keyboard shortcuts is this about as they can have their own effect in a different use case ? Like in the location bar it allows to cycle trough the search engines and the keyboard shortcuts article linked above use it to (un)mute the audio in the media player.

I don't understand how does it work in the address bar, I can change default search engine without holding ctrl just pressing up/down arrows

You can't add new or modify existing keyboard shortcuts via userChrome.css as this requires JavaScript and can't be done via CSS.

You haven't answered my question about what use case of ctrl + ↑↓ your question is about.

What purpose of those keyboard shortcuts is this about as they can have their own effect in a different use case ? Like in the location bar it allows to cycle trough the search engines and the keyboard shortcuts article linked above use it to (un)mute the audio in the media player.

I'm using it for scrolling through the page