You can change key bindings in Firefox with AutoConfig, without having to unpack and modify the Firefox binary.

Create a config-prefs.js and config.js file:

on Windows:

• C:\Program Files\Mozilla Firefox\defaults\pref\config-prefs.js
• C:\Program Files\Mozilla Firefox\defaults\pref\config.js

on macOS:

• Firefox.app\Contents\Resources\config.js
• Firefox.app\Contents\Resources\defaults\pref\config-prefs.js

on Linux:

• /usr/lib/firefox/config.js (resp /usr/lib64/firefox/config.js)
• /usr/lib/firefox/browser/defaults/preferences/config-prefs.js (resp /usr/lib64/firefox/browser/defaults/preferences/config-prefs.js)

If the above path is not correct, use whereis firefox to find the correct directory on your system.

with the following content:

config-prefs.js:

pref("general.config.filename", "config.js");    
pref("general.config.obscure_value", 0);  
pref("general.config.sandbox_enabled", false);  

config.js:

try {                                                                                                                                                                                                                                                                                               
  let { classes: Cc, interfaces: Ci, manager: Cm  } = Components;
  const Services = globalThis.Services;                                                                                                                                                                                                                                                             
  // Firefox 148+: use browser-delayed-startup-finished instead of
  // chrome-document-global-created (Bug 2002767 changed startup sequence)
  function ConfigJS() { Services.obs.addObserver(this, 'browser-delayed-startup-finished', false); }
  ConfigJS.prototype = {
    observe: function (window) {

          // Place your keyboard shortcut changes here
          // ...
          // ...

    }
  };
  if (!Services.appinfo.inSafeMode) { new ConfigJS(); }
} catch(ex) {};

After modifying these files, you always have to go to about:support and run Clear startup cache, to restart the browser with the new config.

Some examples what you can do:

Put these snippets into config.js, where I wrote Place your keyboard shortcut changes here.

Remove an existing keyboard shortcut

// remove Ctrl-Shift-X, so that I can map it to 1Password in the 1Password app later
let keySwitchDirection = window.document.getElementById('key_switchTextDirection');
keySwitchDirection.remove();

Change an existing keyboard shortcut

// remap Ctrl-J to downloads (removing it from focusing the browser bar)
let search2 = window.document.getElementById('key_search2')
search2.remove();
let openDownloads = window.document.getElementById('key_openDownloads')
openDownloads.setAttribute("modifiers", "accel");
openDownloads.setAttribute("key", "J");

Create a new keyboard shortcut

// create keyboard shortcut to Toolbar > Settings with Ctrl-,
let mainKeyset = window.document.getElementById("mainKeyset");
let settingsKey = window.document.createElementNS('http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul', 'key');
settingsKey.setAttribute("id", "key_Settings");
settingsKey.setAttribute("modifiers", "accel,shift");
settingsKey.setAttribute("key", "U");
settingsKey.setAttribute("oncommand", "openPreferences()");
settingsKey.addEventListener('command', this, false);
mainKeyset.appendChild(settingsKey);

Something completely custom. (Firefox introduced the Shift-Ctrl-T as an universal undo (change set), so the example here is not necessary anymore. I'll leave it here to serve as an example for other customizations.)

// make Ctrl-Shift-T reopen last tab OR last window, depending on which one was closed last
let undoCloseTabKey = window.document.getElementById('key_undoCloseTab');
undoCloseTabKey.removeAttribute('command');
undoCloseTabKey.setAttribute('oncommand', 'undoCloseLastClosedTabOrWindow()');
undoCloseTabKey.addEventListener('command', this, false);

window.undoCloseLastClosedTabOrWindow = function() {
  // don't have any tabs to restore
  if (SessionStore.getClosedTabCount(window) == 0) {
    // we don't need to worry whether there are any windows to restore - undoCloseWindow does that for us
    window.undoCloseWindow();
  }

  // don't have any windows to restore
  else if (SessionStore.getClosedWindowCount() == 0) {
    // we don't need to worry whether there are any tabs to restore - undoCloseTab does that for us
    window.undoCloseTab();
  }

  // restore whichever was closed more recently
  else if (SessionStore.getClosedTabData(window)[0].closedAt > SessionStore.getClosedWindowData()[0].closedAt) {
    window.undoCloseTab();
  } else {
    window.undoCloseWindow();
  }
}

Instructions:

To find existing keyboard shortcuts:

• Hamburger menu > More Tools > Browser Toolbox
• In the Inspector, search for #mainKeyset
• There, you see all assigned keyboard shortcuts

To find menu actions that you can trigger with keyboard shortcuts

• Hamburger menu > More Tools > Browser Toolbox
• Use the Inspector to find the element that you want to trigger with a keyboard shortcut, for example appMenu-settings-button
• Take the oncommand attribute from the menu item, and use it as oncommand attribute for the key tag

or

• Use the Inspector, search for #mainCommandSet
• and take the commands's id, and use it as command (not oncommand) attribute for the key tag.

Defining keyboard shortcuts:

• specify modifiers with the modifiers attribute. You can use accel (for Ctrl), shift and alt
• specify the key itself with the key attribute

Source for all of this: Reddit, u/aveyo, Restore Ctrl+Shift+B = Library by setting config.js

More details: Firefox Keyboard Shortcuts (Web Archive)

Related to zackhalil's answer:

For those on Linux or simillar Unix/Unix-like systems replace Firefox64 for /usr/lib/firefox. Don't be confused with $HOME/.mozilla/firefox