I'm using Legacy Vector on a WMF wiki via Firefox 128.0.2
Mediawiki:Sitenotice is not displayed even after clearing cookies and changing Sitenotice ID.

The problem is the following CSS property:

#minerva-dark-mode-launch-banner, .vector-limited-width-popup, #wll2021, body > #centralNotice, #mw-head-base, #wmde-banner-app, .cnotice, #frb-inline, #frb-nag, .mw-dismissable-notice, .cn-fundraising, #centralNotice, #siteNotice, .banner {
  display: none !important;
}

I cannot determine what loads this CSS, it seems to be included as data:text/css.
Thank you!

Edit: Resolved - AdGuard