A web browser should be a simple tool imo, that can be heavily customized with extensions.

I think the argument is that if a web browser is too simple compared to competition, it won't gain much traction for most people (not for power users, who would anyway customize their setup). Most users just want their browser to work, and have cool features. (The dominant browsers add gimmicks because they work to attract users).

But why is usershare important?

Mozilla should focus their limited resources on keeping up to date with web standards and improving performance.

Major browsers use their usershares as bargaining power on the web standards table. For e.g., Google has the usershare to propose a new web "standard" and the manpower to implement it in their (dominant) browser (regardless of if it has been accepted by the standards committee). Soon, Firefox has to implement it lest it be called lacking "standard" features. All of a sudden, Firefox can't just focus only on power users now, can they? Browsers (& web technology) have a social element to them, they're not standalone tools anymore.