Typically, you have to jump in with both feet if you want to shape web browser features -- you have to use early (read: buggy) releases and leap into the developer community. But what if you're happy with a stable version and just want to offer a bit of constructive criticism? Mozilla will soon have you covered. It's launching Idea Town, an opt-in program that lets you try upcoming features and offer feedback. You won't have to ditch a regular copy of Firefox, and these will only be features that are likely to show up. While this means that you won't get strictly experimental features (think Chrome's flags), it should put more of the development process in your hands.
Idea Town won't exist as more than a teaser site when it's officially announced around August 11th (right alongside Firefox 40), but the first real code should start arriving in late summer and the fall. It should be widely available when Firefox 42 shows up, which is tentatively slated to arrive on November 3rd. That's a long time to wait, but patience could be a virtue if you want to transform Firefox without knowing a lick about nightly builds or bug reports.