It turns out that Mozilla was not kidding.
Firefox 5 is already in the works and can actually already be downloaded as a preview.
There are four development stages in Mozilla's new development process - mozilla-central, experimental, beta and final. We know that Firefox 5 has been in development already, but we only recently noticed that the browser has already been posted on the company's FTP server in nightly build form.
The new browser carries the version number 4.2a1pre, which is apparently due to time constraints that did not allow Mozilla to change its versioning system yet. However, there has been a confirmation that this is, in fact, the foundation for Firefox 5, while we also know that there will be a 4.0.x maintenance release for Firefox 4 with minor bugfixes.
According to the release plan, Firefox 5 final is scheduled for release in the week of June 29, while the first stable experimental release of the browser with an actual 5.0 version number should be available in the week of April 13. The expected interface changes, which will include the web application tabs, will not land until early may, however.