While HTTP/HTML isn't quite 20 years old, Tim Berners-Lee first released his Information Management: A proposal dissertation in March 1989—which would later come into full fruition when he put together HTML over HTTP. Berners-Lee idea was simple, create a way to share information at CERN so that information wouldn't get lost with the high turnover. See, the average stay of scientist/researchers at CERN was just 2 years, so lots of ideas and projects would get lost when researchers would leave. He proposed a way for researches to store their documents online in a format that would be easily viewable by everyone.
Who would have thought this paper would have turned out to be what it is today.
It appears that Mozilla has decided to reversion 3.1 to 3.5 due to the sheer number changes that have been made. The current Firefox v3.1 Beta 3 is the last version that will be labeled v3.1. There's a 4th beta planned which will be label Firefox v3.5 Beta 4.
"The increase in version number is proposed due to the sheer volume of work which makes Shiretoko feel like much more than a small, incremental improvement over Firefox 3: TraceMonkey, video tag and player support, improvements to user controls over data privacy, significant improvements in the web layout and rendering platform, and much more," said Mozilla's Mike Beltzner in a blog post last week.
This is probably a good marketing idea and I do think there's enough changes in 3.1 to warrant a higher revision number. I suspect most people would have thought that v3.1 was a very minor upgrade, when in fact there's a lot of significant changes—even if most of them are behind the scene (such as TraceMonkey, Web worker thread support, improvements to the Gecko rendering engine, etc.)