Major releases break the API, so programs that depend upon them may not work. Minor releases add or improve features while preserving existing functionality. Patches fix bugs.
This is exactly why having version numbers is important. Even if the number becomes arbitrary, it's existence is necessary to build dependency trees.
Aside: I'm not certain, but it seem as though Firefox does have a major.minor version scheme: my user-agent reads "Mozilla/5.0 ... Firefox/25.0"; Firefox 5.25 is probably the most correct.
Most, if not all releases of Firefox have minor API changes. Sometimes it's visible via HTML, sometimes they are only visible to plugins. By semantic versioning, 25.0 is the correct version.