Is your Firefox not responding? Or are you regularly seeing the "Firefox is running but is not responding..." dialog? This usually means that something in Firefox is hung. This may be because of a code error within Firefox itself, such as a deadlock or infinite loop, or it may be caused by 3rd-party software such as a Firefox extension, antivirus software, or even malware or a virus on your computer.

What information to include in a bug report

• As usual, following Bug writing guidelines will make your report much more likely to lead to a fix in Firefox.
• Check the CPU usage of the Firefox process(es): is it idle, using 100% of a core (which may show up as 50%, 25% in your monitoring tool if you have a multi-core processor) or neither?
  • (If you're experiencing high CPU usage and Firefox eventually recovers from a hang, you should try the instructions at Reporting a Performance Problem instead.)
• Is the rest of your system busy (high CPU or memory usage, or high disk activity)?
• If Firefox is truly hung, please note the approximate amount of time you waited before forcing a crash (below) in the bug report. If you suspect a hang (as opposed to slow performance), wait one minute on an idle system before getting the stack trace.
• The crash report link for a hang, obtained by following the steps below.

Submit a crash report for a hung Firefox

The easiest way to help the Firefox stability team to debug these kinds of hangs is to make the hung Firefox crash and submit a crash report. Then file a bug report with the crash report ID. The steps to induce a crash are slightly different on each platform:

Windows

On Windows, the stability team has written a utility which will inject a crash into Firefox which will trigger the crash reporter: Download crashfirefox.exe here (source code). Running this tool will search for firefox.exe and inject crashing code into it. The tool also has command-line options to kill processes with other names or by process ID.

• Run the crashfirefox.exe tool.
• The Firefox crash report UI should appear. Please check the box to send the crash report to Mozilla
• Start Firefox. Type "about:crashes" in the URL bar and click on the top listing. The crash report should look something like this report, which is an example of bug 1132241.
• Create a new bug report at bugzilla.mozilla.org using this link.

Linux and Mac

On Linux and Mac (Firefox >=27), sending SIGABRT to a process will trigger the crash reporter. You can use any process monitoring tool to find the Firefox process ID (PID).

Then, in a terminal, kill Firefox using a command such as this:

# Replace 31042 with the PID of Firefox found above
kill -n SIGABRT 31042

# on osx
kill -s SIGABRT 31042

• The Firefox crash report UI should appear. Please check the box to send the crash report to Mozilla
• Start Firefox. Type "about:crashes" in the URL bar and click on the top listing.
• Create a new bug report at bugzilla.mozilla.org using this link.

All OS

As long as you do not need to be in Firerfox's safe mode, a user friendly method of crashing Firefox is to install and use the addon Crash Me Now! (Simple)

• Use the menu button to find the Crash me icon and click the icon.
• Firefox crash report UI should appear. Please check the box to send the crash report to Mozilla
• Start Firefox. Type "about:crashes" in the URL bar and click on the top listing.
• Create a new bug report at bugzilla.mozilla.org using this link.

Another way to trigger a crash in a hung tab is to run the following script in the Browser Console (opened after selecting the hung tab):

let wm = Cc["@mozilla.org/appshell/window-mediator;1"].
         getService(Ci.nsIWindowMediator);
let win = wm.getMostRecentWindow("navigator:browser");
let browser = win.gBrowser.selectedBrowser;
if (browser.isRemoteBrowser) {
  browser.messageManager.loadFrameScript('data:,let appInfo = Components.classes["@mozilla.org/xre/app-info;1"];if (appInfo && appInfo.getService(Components.interfaces.nsIXULRuntime).processType != Components.interfaces.nsIXULRuntime.PROCESS_TYPE_DEFAULT) {Components.utils.import("resource://gre/modules/ctypes.jsm");var zero = new ctypes.intptr_t(8);var badptr = ctypes.cast(zero, ctypes.PointerType(ctypes.int32_t));var crash = badptr.contents;}', true);
}

Other Techniques

On OS X if you use a Nightly build (>= Firefox 16), you can use Activity Monitor's "Sample Process" feature to generate a sample. This won't produce useful results on a regular release/beta build.

Instead of crashing Firefox, more advanced developers who know how to use a debugger can try attaching to the Firefox process and debugging in place. This is only useful if you have Firefox debug symbols, which are available on Windows from the Mozilla symbol server.