Mainly because it's not a Chromium based browser. Chrome is making it hard/impossible to use adblock, and if Firefox were to die for whatever reason, Google would basically monopolize the way we interact with the internet (which it has almost done already)

uBlock Origin (the good ad blocker) works best on Firefox because Google made it impossible for it to work as well on Chrome, and all browsers based on Chrome inherit that flaw. uBlock Origin is better because the developers don't have an "acceptable ads" program where they let some ads through, like Adblock Plus does.

Also, the developers of Adblock Plus let websites buy their way into the Acceptable Ads program.

https://ublockorigin.com/

https://adblockplus.org/en/acceptable-ads

https://restoreprivacy.com/report-ad-blockers-allowing-acceptable-ads/