Firefox has been rock solid stable for me for years, and I also questioned why people were complaining so much about stability.
That changed in (if I remember correctly) version 33 on Windows. I've had nothing but problems with it ever since, regardless of which plugins or extensions are installed. In particular, I've noticed that if the browser is open overnight, opening any menu will cause an instant crash. Frequently, right-clicks that bring up a context menu will also cause a crash, but only if the browser has been open for a while.
I have multiple installs of the browser for development purposes, including a "stock" profile with no plugins or extensions installed. I'm pretty sure the crashing is not because I'm holding it wrong.