It may have been the best way to deliver video in the early days of the internet, but the rise of standards compliant technologies make Flash look obsolete. More importantly, however, Flash is riddled with holes, and it's often used as a beachhead for hackers to get comfortable inside your computer. Oftentimes, this is met with silence from Adobe, a point that The Register put to the company's Wiebke Lips. Her response was that there are "extensive efforts" in progress at the company to make the code harder to crack, although they're not yet ready to be pushed out to users.
2015 is becoming the year that Flash gets killed-off once and for all, with Google tweaking Chrome to "intelligently" block auto-playing adverts. In addition, YouTube, the site that was probably the plugin's biggest proponent, switched to HTML 5 to deliver video at the start of the year. It's likely that other notable holdouts will be pressured, either by these latest moves, the most recent security controversies or the Occupy Flash movement. Yup, there's a whole movement dedicated to eradicating this stuff from the web.
Oh, and should you want or, more likely, need to use Flash, you can reactivate the feature at your liberty by dipping into Firefox's settings menu.