In between Firefox 7 and Firefox 10 we improved performance, shrunk memory usage, killed leaks, vastly improved hardware accelerated rendering for the canvas element, improved Firefox Sync's setup & reliability, vastly improved the way that add-on compatibility is handled, added the ability to load tabs on demand during a restore, added Twitter search, made the audio and video elements far more reliable, improved our theme integration on OSX Lion, we hide the forward button when it's not useful, fixed a ton of security and crasher bugs. The browser actually looks, feels and acts differently from 7.
For developers we added text-overflow-ellipsis, WebSockets, web timing, improved MathML, added support for displaying images in WebGL (via CORS), HTML5 context menus, added support for insertAdjacentHTML, added support for hyphens in CSS to support more than just English, shipped our new TI JS engine (much much faster!), added hooks so you can read DNT state from JS, added support for font-stretch, added support for anti-aliasing in WebGL, CSS 3D transforms and Mozilla's HTML5-based full screen API. We made IndexedDB much faster, shipped a bunch of new web developer tools and a huuuuuuuge pile of things that I haven't listed here.
So we've been busy and we like rolling those positive changes out to users as fast as possible. That's why we're doing new releases and bumping the version number quickly. It's not just for marketing. It reflects real change over time.