I think what a lot of comments are missing is that the value of a browser is directly boosted by the value provided by its extensions.
I wouldn't have switched to Firefox from Chrome if most of the extensions I was using wouldn't have had readily available equivalents or workarounds. I would have held off on test-driving the new Edge as my daily workhorse browser if I hadn't noticed an extension for my password manager in the Microsoft Store and found that I can install most extensions from the Chrome Web Store too.
Netscape and Opera prove that the paid browser business model is dead, sure, but that doesn't mean browsers (especially browsers developed by companies with lots of related paid services) don't represent value for the companies developing them -- and that those companies don't profit from that value.
Google especially benefits massively from Chrome's marketshare -- even to the extent that their own web services can get away with treating competitor browsers as second-class citizens. Extension developers contribute to the value driving that marketshare and open source extension developers do so with practically no return on investment.
This is just another example of the true sharing economy of open source clashing with capitalism (which is inherently based on extracting surplus value as profit, not sharing it back). Which would be fine, really, if the people participating in the sharing economy wouldn't also exist in capitalism and have to rely on their success under capitalism to ensure basic subsistence like food, shelter and the means of extension development.
Extension developers using ads, tracking, malware or selling their extensions to malicious entities are just trying to find ways within the system to capitalise at least on some of the value they've provided. That's undesirable and developers shouldn't be in such dire circumstances to be willing to give in, but this is a systemic problem.
Luckily unlike with most systems, this system is almost entirely in the hands of browser vendors. Browser vendors could emphasize donation options -- just look at how big open source projects like some Linux distros push donations while still allowing for freeloading. But browser vendors currently have no direct incentive to do so -- in fact, doing so might actually harm their metrics.