Brave is not funded by an ad company like Google.
Because they are an ad company themselves, no ad company is going to fund them, they are competitors.
Apart from that Brave is dependant to Google way more. More than 90% of their browser’s code is by Google developers. They don’t even have their own store lol and distribute extensions in Brave through Google’s chrome store!
That’s independence? Calling Google for something basic like installing extensions to your browser?
I prefer Mozilla’s and Vivaldi’s model. They both get money from their search deals (Mozilla with Google, Vivaldi with Microsoft) but they are not ad companies themselves.
Yes, I wouldn’t use a browser from an ad company, no matter the size of the company.

Maximum privacy many times comes with compatibility issues and pages which don’t properly.
Brave is an ad company, so they are interested in blocking the ads of other ad companies, that’s their business and that’s why they do it.
Firefox is not a browser from an ad company, so what they offer out of the box is a browser with the less compatibility issues possible, and that’s not an easy task when they use their own engine.
The job to block ads etc is done by extensions in Firefox and since Firefox is not an ad company it makes sense for them.
They have ublock origin as a recommended featured extension.

Nice try by Brave. But they only can fool their fanboys.
Brave sells ads, so they block the ads of their competitors by default.
Firefox is not an ad company to block the ads of other ad companies out-of-the-box.
We have ublock origin for that.