Unfortunately Firefox wanted to outplay IE by releasing this as a major version upgrade. The number game is important and I am sure they felt threatened by the IE9 release which is a huge step up for Microsoft.
People might not care about the version but companies regard version upgrades as a huge matter. Minor upgrades are just routine upgrades to the IT infrastructure. But when it comes to major Version upgrades you start getting management involved who then decide where to take it. Besides switching to Version 5 for no apparent reason as there are no major upgrades to the browser, they are now dropping support for 4. This is a sad, sad time for Firefox. It is a stupid move trying to out-smart the competition. They played it so well for such a long time. But you do not want to piss off the corporate world. This is how Microsoft became so popular in the first place (Active Directory, Exchange, IE.6 etc - they are long lasting).