Windows/Mac/Linux: Mozilla took the wraps off Firefox 10 today, and despite it being a nice round version number, this version of Firefox is in line with Mozilla's new Chrome-style rapid release schedule. The new version now automatically hides the "Forward" button until you browse back on a page and have a use for it to keep the UI trim, supports full-screen APIs for web apps and utilities that can use the entire display, and improves the way you manage installed browser extensions.
Aside from this, the bulk of the update are bugfixes, speed improvements and under the hood tweaks. Firefox 10 also marks the introduction of an enterprise-friendly "Extended Support Release", which will see only security improvements and bug fixes until the updates in the consumer version are large enough to warrant a large-scale update to the business version. Mozilla has this on a "one business update per seven consumer update" cycle, so if Firefox is updating too quickly for your liking, the enterprise version may be for you.
You can snag the new version of Firefox now at the link below, or use the browser's in-app updater to download it.