I don't have a problem with it, though I am wondering why they aren't simply calling newer builds of Firefox, which mostly seem to involve just changes to the under the hood functionality of the browser, another sub-version of a version of Firefox; eg Firefox 4.5. I like the changes they're making, though their versioning I would like to see reverted back to the old method rather than mirroring a chrome release.
I like how the Mozilla team is speeding up updates, but I dislike the fact that they decided to use 5.0, 6.0, 7.0, etc...
Point releases like 3.6 were considered a major release in the past, so using numbers like 4.1 and 4.2 would have made much more sense, especially for those who aren't aware of what's going on and would expect lots of changes with a jump from 4.0 to 5.0. In fact, there really aren't many changes at all and most (if not all) are things most users would not care about or even notice.
Versions like 5.0, 6.0, 7.0 should be reserved for the truly big updates, like major new features (such as the Panorama feature introduced with 4.0) and interface changes.
Download the Mozilla Add-on Compatibility Reporting extension. It actually disables the check and allows you to mark it as "Still working" or "Broken" for others to know about. Since I frequently run a beta/nightly version of Firefox, it comes in handy.
Its alright it is good to see firefox evolving again but like many others I dislike the marketing gimmic of 5.0, 6.0, 7.0 when it should be 4.3, 4.4, 4.5, ect. a major release like a front end update should be a major version number not a refinement release like most of the recent ones.