> If scrappy Firefox

Because ie at the time was dogshit. FF was such an indisputable improvement that people just had to switch.

Chrome great. There is nothing a newcomer can do to compete.

There is plenty a browser could do to compete, from blocking modern popups (which now occur within the window, putting a gradient over the content forcing you to interact with their "subscribe to our mailing list" or "join our site to socialize" prompt that you would have to waste time clicking past, auto-pausing looping videos in unfocused tabs, throttling crypto mining tabs, providing uBlock Origin, handling WiFi Terms of Service click through, etc.

Think of the plethora of instances where you wish your browser made minor changes to make browsing easier

There are plenty of things to compete on: efficiency (startup time, memory use), security (from ad blocking on sites to extensions to the app itself), customization (both in looks and behavior)