FIrefox USIng ONions
wiki.mozilla.org
wiki.mozilla.org
Many public Internet websites filter connections from the Tor network, many other websites are very slow, yet others impose extra obstacles such as multiple rounds of captchas (even 5 or more) or degraded service (including high suspicion of payments), and of course you often will receive webpages in the wrong locale or language - which can trigger regional filters. Currently, workarounds requires resetting the circuit (few non-technical users will even understand what the circuit is), lots of patience and reloads, and often just giving up.
If that's the experience of typical Firefox users, they won't use it and they will have bad associations with Tor and Firefox.
That seems like a long game. Tor is starting with a minuscule marketshare; Fusion will only be used by users in a special mode, afaict - again, probably very few; and if Brave's marketshare is higher or lower, the difference is immaterial. There are even websites that aren't compatible with Firefox itself, or so I'm told; it will be a long time before anyone cares about Tor users.
Also, users whose personal data is harder to access are less valuable to the website; websites have an incentive to make it harder to use Tor. Finally, many associate Tor with criminality; that's not an attractive market and it's very easy for the CSO to say, 'just block it; it's not worth the time or cost'.