I thought you were joking about Firefox 7. From version 4 to 7 in one year, are you kidding? It turns out you're right. For some reasons, I really dislike software companies jack up version numbers: how can you have FOUR MAJOR releases within one year for a relatively complex software? That gives me an impression that they feel insecure of becoming irrelevant, instead of competing on features and usability, they choose to compete on version numbers.
I could be wrong, but I just don't feel like they fall into the same trap Netscape did a decade ago.
I could care less, but companies have done it several times.
Netscape skipped version 5 (a doomed project).
Microsoft ditched version numbers for years (95, 98, 2000), and then just names -- due to how version number increments on an existing product appear to end users.
To technically savvy it doesnt really matter "is it 5.0 or 4.5?"
But the version number or name is about trying to position the product as a "new one" or even just "mature" to the more typical end user.