It's yet another thing to install & configure manually. For a browser that boasts about privacy at every possible opportunity I'd expect something as standard & essential (in this day and age) to be included.
Once. Coming up on 5 years ago now for me. Every time since, I log into sync, and it appears with all of the proper configurations already set up to my preference.
It shows a clear case of mission statement in conflict with business model and shakes my confidence of Mozilla as the champions of privacy and freedom they paint themselves as.
Keep in mind that they don't even need to make it built in, they could instead bundle ublock origin as a preinstalled add-on and provide a little bit of money to the developer. It would simultaneously improve the security and browsing experience of many of its users while attracting many new ones and funding the fight against invasive online advertising.
But big daddy Google would get so cross then and Mozilla would be forced to adopt an actually privacy respecting alternative like duckduckgo and lose a lot of their funding.
I don't want to main Brave as I currently do, both because of its use of chromium and my ill feelings of the CEO's donations to political movements against the LGBT community but Mozilla management have lost my confidence over the years despite the actual deveopers remaining world class.
I don't think Google would be as mad as every other publisher on the internet who relies on ads for their living. They would go on campaigns to block Firefox from their websites entirely. They already rail against adblockers with full-site popups and blocking content until you disable your adblocker, making adblocking a default would escalate the situation exceptionally quickly.
Firefox's marketshare would drop even more quickly if people couldn't visit their favorite sites with it (and were likely directed to Chrome or Edge).
Without an alternative, readily available method of monetizing websites, it's realistically going to have to remain opt-in (i.e. installing uBlock).
This whole argument is largely invalided by the mere existence of Brave anyway. You could argue that their crypto based mode of monetisation is not the absence of rather the reimagining of ads but regardless of how you feel about crypto, it's definitely superior for the end user and offers effective monetisation that is optional.
Also, you can choose to disable all the crypto and still get the built in adblocking. With Firefox, you can only turn to add-ons.