Tiers

From MDC

The word "tier" refers to a level of organization in a system. Every system can be decomposed into serveral pieces, each of which can be futher decomposed in order to reach a deeper knowledge of it.

Every system can be organized following multiple criteria, thus providing several models which complement each other.

The word tier is used in two different contexts in Mozilla.

Build Tiers 

Build Tiers are portions of the source tree that projects can include or omit as needed.

Supported OS Tiers 

This term refers to levels on which there are more or less support for some combinations of Operating Systems, Platform and Compiler. You can find the 3 defined tiers in the Mozilla Build FAQ.

This might be not an exhaustive list of uses for the word tier.