Both Brave & Opera have built in adblockers that are not dependent on Manifest to run. I haven't played with Opera too much, but Brave lets you add custom lists and works quite well. Combine that with a DNS based adblocker such as HaGeZi [1] or OISD from free DNS providers like ControlD or NextDNS and you'll be golden.
No, and it shouldn't. Firefox is a general purpose browser, so it should display web pages as the author intends. Security/anti-malware is expected, but changes in content are outside its scope. That's what extensions are for.
If you're installing Brave or Opera, you're not interested in a general purpose browser. Adblock is part of their advertised feature set.
[1] https://github.com/hagezi/dns-blocklists?tab=readme-ov-file#...