Because it is slow to start, and can have weird quirks where things don't work quite right. You also have less control over when it updates, which doesn't both

Because it is slow to start, and can have weird quirks where things don't work quite right. You also have less control over when it updates, which doesn't bother me that much, but is a problem for some people. It also takes up more space on disk, and it is the snap packager's responsibility apply security updates on upstream dependencies, rather than the ubuntu packagers, although for Firefox, I don't think there is too much worry there, as long as Mozilla puts some effort into the snap.