Browser Speed

The speed of the browser plays a very big factor in the browsing experience. If the browser is slow, then you are going to face slow loading speed on websites and will also have problems in switching tabs.
Firefox browser is very much fast in terms of speed. The browser blocks the web trackers which store your cookies while you are surfing on the web. By disabling the web trackers feature, your webpages will be going to load so fast and smoothly.
Brave is a lot faster than any other browser. The main reason is that this browser blocks all the third party ads, which makes the sites load very much faster, and it also disables all the cookies in the browser, which makes it more functional.

The performance on the Firefox browser is very much up to the mark. Firefox has all the features that can make a browser’s performance best. It also works very fine with smartphones having very limited system capabilities.
Brave is one of the lightest browser available on Android platform. This helps the browser to perform fast and give better performance to the user. It also has features like battery saving and data saving modes, which helps the browser to enhance its performance. Their official website has also mentioned that the browser saves more than 18% of bandwidth.
Privacy and Security

Firefox provides the best privacy and security for its users among any other browsers. You can change various privacy and security features in Firefox. You can also enable the option of Tracking protection by which the web trackers installed on the website can’t recognize you.
Brave is also very strict in terms of Privacy of users. This browser has several different privacy features, and it gives control to all of them. Some of them are blocking websites, changing ads preferences, and disabling cookies.
User Interface

The user interface of Firefox is very smooth and gives a very great browsing experience. You can see the option of history and bookmarks direct in the Home tab, which makes it so much easy to access them. If you use so many tabs, then switching between tabs is also simple.
Brave is a simple browser in terms of UI. On the Home tab, you see a search bar along with some web links, a wallpaper, and some different toggles at the bottom to do other functions. It is not that much advance, very simple, and responsive, which makes it work fast.
Conclusion
Both of these browsers work perfectly fine on Android. If you want a more performance-based browser, then Firefox can be the option for you. But when it comes to privacy and clean UI, then Brave is the one which should be preferred.
The speed of both browsers is almost the same, but brave is slightly better.