Mozilla released the next update for Firefox on Thursday, July 2nd (release notes show the original planned release date of June 30th) with the Firefox 39.0 release. There are various OS related changes and fixes, the complete details can be found in the Firefox 39.0 Release Notes.
Depending on their update settings, users will be prompted to update within the next 24-48 hours. Users can also manually update by going to the Firefox Help Menu and selecting About Firefox and follow the prompts to update. Alternatively users can also down and manually install the update via getfirefox.com site.
The next planned release for Firefox is Firefox 40 on August 11, 2015.