On huge project, like Firefox, Seamonkey, Iceweasel, Glib,  etc..., compile time can take a very, very long time. If you are a proud owner of a dual-core or multi-core processor, you can reduce the compile by increase the number of simultaneous jobs that make can have.

The process is simple. Usually you would:

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./configure
make
make install

but you should do (for example: for dual-core's) :

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./configure
make -j 2 # replace 2 with the number of cores of your processor.
make install

Although on small projects that compile themself in less than 20 seconds this wouldn't change things a lot, this will reduce considerably the compile time for all other projects.

Enjoy!