In this guide, we'll show you a few different methods for fixing the Firefox is already running but is not responding error message on a Linux system.
First, let's go over a few reasons for why this error may be occurring. It's not uncommon to have an application freeze or "hang" every once in a while, so if this isn't an issue that you repeatedly have with Firefox, you can probably simply kill the process and get back to what you were doing.
This is a companion discussion topic for the original entry at https://linuxconfig.org/how-to-fix-firefox-is-already-running-but-is-not-responding-error-message