The browser is a succesful product in the sense that it has found product market fit: there is a group of users that value the key benefits of firefox (not large company controlled, opensource, focussed on security and privacy). It may not be a large group of users, and it may not be a growing group of users, but it has a group of appreciative users that would rather use firefox over other browsers.

So it's a niche product targetted at a niche user group.

Where firefox the company is failing is to have a financial sound way of delivering that niche product. To keep the product relevant it needs to invest, and it needs some revenue to pay for these investments. This could be through other products, sponsering, subsidies, etc.

As a long time firefox user i care about having an alternative browser. I don't care about the company mozilla, nor any of its other products. But i understand the money to keep firefox relevant needs to come from somewhere.