Sometimes when you try to start Firefox, it warns you that Firefox is already running. The message looks like this: Firefox is already running, but is not responding. To open a new window, you must first close the existing Firefox process, or restart your system. Usually, you can just kill the firefox process to solve this problem. Mozilla Firefox is an open-source web browser developed by Mozilla. Firefox is available for Windows, macOS, Linux, and Android. Firefox has been the second most popular web browser since January, 2018. Firefox uses the Gecko rendering engine. Download Firefox Firefox is regularly built for the current branch of the operating system, the latest versions are packaged for each release and remain frozen until the next release. In 2017, Landry began hosting packages of newer Firefox versions for OpenBSD releases from 6.0 onwards, making them available to installations without the ports system.