Mozilla announced the Firefox 3.5 release candidate on Friday, June 19, 2009. The Firefox 3.5 Release Candidate is a public preview release intended for developer testing and community feedback. It includes many new features as well as improvements to performance, web compatibility, and speed.

New features and changes in this milestone that require feedback include:

- This beta is now available in more than 70 languages
- Improved tools for controlling your private data, including a Private Browsing Mode.
- Support for the HTML5