A memory leak can occur anytime a software program is changed, updated, or added on to. The issue isn't with Firefox the Program per se, but to the individual Firefox installation. Firefox (or Chrome) installs and runs just fine on millions of computers, but because it is one of those programs that is constantly updated, and allows many user configurations there is always the possibility that the installation can become corrupted. That's when you get the leak.
And it makes sense that a theme add on would do it.
Just keep in mind that when you observe the behavior (with any program - not just Firefox) that the first course of action should be re-installation - or if available in the installation package, a repair install. Or as in your case - remove the offending module.
More!: Memory leak - Wikipedia, the free encyclopedia