This has always been the case for years, for every browser. No idea why you say "now".
Any browser providing extensions, not just Firefox, will at least won't allow extensions to modify their own extensions providing page. Otherwise, any malicious extension can infiltrate and trick users to download modified extensions from the browser's own extension page.
uBO Lite is exactly MV3, it has most of MV3 limitations plus its own limitations comparing to Chromium. Safari just auto converts MV3 codes to Safari's own blocking engines.
Any browser providing extensions, not just Firefox, will at least won't allow extensions to modify their own extensions providing page. Otherwise, any malicious extension can infiltrate and trick users to download modified extensions from the browser's own extension page.
Comically ignorant.