These are the Firefox hotkeys combo if they are not listed or user edited those settings with Addon or 3rd party app then Firefox hotkey default will not work until that is removed.

https://support.mozilla.org/en-US/kb/keyboard-shortcuts-perform-firefox-tasks-quickly

What keyboard layout do you use that you can't click the ']' key? Do not get confused by the "Ctrl+Shift" modifiers as normally the best matching character is specified in the source code. Compare with zooming: Ctrl+0 and Ctrl+- and Ctrl++.