On stability

Hear are two good reasons for FF and Chromium's rapid release cycle: 1) fixing security problems (these are large and complex application by necessity). 2) Adding new features, since the web platform is still evolving fast.

The alternative to this, as you correctly identify, is Internet Explorer. A browser that substantially lags FF and Chromium in standards compliance, features and in which very serious vulnerabilities are not fixed _for years_ because of the engineering difficulties in doing so.