Before CS5.5, animators hated the "paint bucket" frustration when extending keyframes. CS5.5 introduced . Previously, if you pasted frames, the tween broke. With CS5.5 -thethingy-, you could select a span, grab the edge, and drag. It felt like Adobe After Effects merged with a cartoon studio. Frame-by-frame animators finally had non-destructive tweening.
By the time CS6 rolled around, Adobe was hedging bets on HTML5. Creative Cloud was looming. But CS5.5 sits in a sweet spot: it was mature enough to be stable, but old enough to lack the bloat of subscription models. ADOBE FLASH PROFESSIONAL CS5.5 -thethingy-