Firefox 1.5 has been released! If you've been using Firefox 1.5 Release Candidate 3, congratulations: you already have Firefox 1.5.
Power users upgrading from Firefox 1.0.7 will appreciate draggable tabs, a much faster Back button, and the replacement of many error dialogs with error pages. Web developers and can now take advantage of SVG, Canvas, E4X, and several new CSS selectors and properties. See the release notes or my detailed changelog for more about what's new.
Equally important for security is an overhaul to the software update system. From now on, security updates will be small binary patches, usually between 200KB and 500KB. These updates can be installed with one click, in contrast to using the new version's installer (like you would to update from Firefox 1.0.6 to Firefox 1.0.7). Update notification has also been improved; the subtle red button in the menu bar has been replaced with a dialog. The result of these changes is that it's now much easier to keep your copy of Firefox up to date.
For an idea of the size of updates, the update from Firefox 1.5 RC2 to RC3 (fixing two bugs) on Windows was 218K, while the update from RC1 to RC2 (fixing about ten bugs) was 390K. Major updates, such as the update from Firefox 1.5.0.x to Firefox 2.0, will likely be bigger but I don't know how much bigger. The binary patch system is based on bsdiff; you can read about the algorithm if you're interested.