After four years of predominance in the new trends of browser market, Firefox seems to have exhausted its power of stimulating new users and adopters. In the meantime, we're assisting to the meteoric rise of Chrome among web users. The point is that during all these years Firefox has not been able to develop new strategies for getting more users.
Supporting web standards is not enough: now even the "infamous" Internet Explorer is on the right way to become a full standard compliant browser as of version 9. Using extensions is not enough: despite the fact that these kind of plugins seem to reasonably affect the overall performance, now other browsers start to adopt a similar strategy. Performance is not enough: even if the overall performance of the newest release seems to put Firefox to the level of performance monster like Chrome and Safari, now all other browsers are adopting the same strategy.
In a nutshell: before Chrome, Firefox was the alternate browser, with a capital "t". Now it seems to be just another alternate browser.