Clearly, they are pulling the old "major version change marketing stunt" to catch up with a perceived lag in version numbers compared to other browsers. You see, apparently people think IE 9 must be vastly superior to FF 4...
Personally, I think the time for version numbers should probably be over altogether. At this point I feel it's more productive to just tack a date string onto the thing and be done with it. For example, Firefox 2011-July is way more informative than an actual version number.
Did you read Mozilla's version number announcement a few months back? Version numbers aren't being used in marketing material anymore, so the user will rarely see them. The change in the version numbering corresponds to a new rapid release/update cycle. From the user's perspective, there are no versions, there is just Firefox.
I didn't read it, no. But looking at the website and the actual Firefox app right now, version numbers are still a big part of the picture. A release cycle doesn't become more rapid just because they're using major version numbers in place of minor ones. I get and support the idea behind a rapid release mechanism instead of holding back a release in order to wait on incomplete features, but still this version number thing seems like a stunt to me that has little to do with the actual release policy.
Personally, I think the time for version numbers should probably be over altogether. At this point I feel it's more productive to just tack a date string onto the thing and be done with it. For example, Firefox 2011-July is way more informative than an actual version number.