Mass update Firefox passwords (run in Tools → Web Developer → Browser Console) - https://developer.mozilla.org/en-US/docs/Tools/Browser_Toolbox#Enabling_the_Browser_Toolbox

updatePasswords.js

function updatePasswords() {
  let oldPassword = prompt('Old password:')
  if (!oldPassword) return
  
  let loginManager = Components.classes['@mozilla.org/login-manager;1']
                               .getService(Components.interfaces.nsILoginManager)
  let matchingLogins = loginManager.getAllLogins().filter(l => l.password === oldPassword)
  let matchCount = matchingLogins.length
  if (matchCount === 0) return alert('No matching logins found')

  let newPassword = prompt('New password:')
  if (!newPassword) return
  let confirmNewPassword = prompt('Confirm new password:')
  if (!confirmNewPassword) return
  if (newPassword !== confirmNewPassword) return alert('New passwords do not match')

  if (!confirm(`Are you sure you want to update ${matchCount} password${matchCount === 1 ? '' : 's'}?`)) return

  // Borrowed from mass_password_reset-1.05-tb+fx/content/oldpassword.xul
  let nsLoginInfo = new Components.Constructor('@mozilla.org/login-manager/loginInfo;1',
                                               Components.interfaces.nsILoginInfo,
                                               'init')
  for (login of matchingLogins) {
    let updatedLogin = new nsLoginInfo(
      login.hostname,
      login.formSubmitURL,
      login.httpRealm,
      login.username,
      newPassword,
      login.usernameField,
      login.passwordField
    )
    loginManager.modifyLogin(login, updatedLogin)
  }
  alert(`${matchCount} password${matchCount === 1 ? ' has' : 's have'} been updated`)
}