After many months of testing, five beta versions and two release candidates, Firefox 3.0 was released in June 2008. The 3.0.x branch was the major stable branch of Mozilla's web browser for a long time, and it's been updated to version 3.0.18 so far. Now, however, it reached the end of the line, as Mozilla's development plans include only one more update for it.
The users who have moved on to the current stable branch, 3.6, are probably wondering what we're talking about, but many users still use the 3.0.x version; with this move Mozilla is indicating that it wants these users to upgrade, too.