The QUAKE LIVE website reveals id Software's plans to transition QUAKE LIVE from a browser-based game to a standalone game by the end of this year (thanks nin via Polygon). They explain that changes to how browsers support the kind of plugins their online shooter requires are dictating this move, adding there are benefits to how things will operate this way, but also give the bad news that this spells an end to the game's OS X and Linux support. Here's why:

We will be unable to support Mac and Linux clients with this transition. While we have reports from our testers that the game works through emulation or virtualization software, we are unable to support native Mac and Linux versions. If you’re using Mac and Linux and have a paid subscription, you will only be able to access the game using emulation or virtualization software.