My theory on the Breen, which will probably be proven wrong by Thursday:
The Breen homeworld has an eccentric orbit around its primary, giving it extremely long, harsh seasons. The Breen have evolved to be able to take two forms, each adapted to a different climatic stage.
The ordinary humanoid stage is adapted to the long summer, which is temperate, while the fluidic/gelatinous form is adapted to the long winter, when the planet Breen is a frozen wasteland. What they're made of I don't know, but it's something that's stable at subzero temperatures and breaks down or evaporates at higher temperatures.
The shift to summer happened only a relatively short time before the 24th century, and it's still summer in the 32nd. This explains the contradictory accounts of planet Breen by outsiders; the tales of the frozen wasteland are out of date.
The winter form is stronger and more resilient than the summer form in the right conditions, but can't survive in normal class-M environments. At some point, the Breen learned to be able to shift between the forms at will, and decided their winter form was superior. They developed their suits to keep themselves cold, just so they can stay in their "superior" form all the time.
Perhaps the ship we saw in "Mirrors" is an environment just for dealing with other species, or perhaps they keep their ships in class-M conditions and stay in their suits all the time out of a cultural imperative.
Eventually, maybe with L'ak's help, they'll learn that neither form is superior, merely adapted to different conditions.
