Mozilla just released a new update to Firefox — the default web browser in Fedora — that resolves a pair of high priority security issues. These updates are now available in the Fedora 21, Fedora 22 and prerelease Fedora 23 repositories. It is highly recommended that all users of Firefox update to version 40.0.3 using the Software application in Fedora Workstation, or using the command
sudo dnf update firefox from the terminal. Note also that these updates are still propagating to all the mirrors, so you might not see these updates yet, but they will appear soon.
The updated version of Firefox resolves the Add-on notification bypass through data URLs and Use-after-free when resizing canvas element during restyling issues, in addition to a handful of other bugfixes.