Memory leaks waxed and waned as development focus changed. I remember Firefox's memory usage being quite decent around version 2, and even into 3. Later, some things got bloated, though a large part of the memory problem was due to misbehaving plugins. The memshrink project managed to fix even memory leaks across plugin authors' projects.

Also, note that a GC does not automatically mean no memory leaks. For instance, see how leaky Gmail is (was worse according to their dev team).