Most users should not notice a difference. All supported configurations which use OpenGL (FirefoxOS, Android, modern OSX) already use OMTC. If you use OpenGL on Linux, however, read on.
OpenGL in Linux
OpenGL on Linux is not a supported configuration (i.e., fixing bugs is not a priority - we would love some volunteer help here, by the way, get in contact if you're keen). However, if you have good luck with your drivers, then it works pretty well and can be enabled by setting the 'layers.acceleration.force-enabled' pref to true. The main benefit is improved WebGL performance. OMTC on Linux also works pretty well, but is also not a supported configuration. If you want to continue using OpenGL on Linux for versions of Firefox 28 and later you will need to use OMTC. (Note that if you are considering trying OpenGL on Linux for the first time, you should use a new profile to make it easier to undo if your drivers are not cooperative. And be prepared for your system to crash, potentially).
Nightly users will automatically get OMTC, if they currently get OpenGL. That is, for Nightly users, setting 'layers.acceleration.force-enabled' on Linux will get you OpenGL with OMTC.
For Aurora, Beta, and Release users (once version 28 hits those channels), you will also need to set the environment variable 'MOZ_USE_OMTC' and the 'layers.offmainthreadcomposition.enabled' pref (as well as the 'layers.acceleration.force-enabled' pref) if you want OMTC. Otherwise, you will get basic layers (software composition, although usually hardware accelerated by X).