Basically if you played Tribes when it first came out almost 11 years ago you will be familiar with the browser based version. It's basically the same game with a few extra bug fixes, according to InstantAction reps. The game is also purely multiplayer (no bots here) so you and your friends will have to join servers and play in the game's massive levels, defend bases, build and repair turrets and items and more. In short it's going to be pure nostalgia fun.
But will there be more to come? While InstantAction already has a kind of spiritual successor to Tribes running on their servers (Fallen Empire: Legions) they did say that if the browser version of Tribes turns out to be a hit they could add new content to the game (the company now owns the rights to the Tribes franchise). There's no release date set for the InstantAction version of Tribes but based on our playtime it shouldn't be too much of a wait.