Firefox error messages can occur when starting Firefox or when trying to load a website. From errors on your computer to errors on your Firefox profile to installation or network errors and even errors with Firefox’s network settings, each error message points to a specific problem and solution.
Firefox is Already Running but is Not Responding
This Firefox error message indicates the Firefox process is still running in the background on your system. Firefox requires exclusive access to your user profile, which contains your personal Firefox settings, and the other Firefox is still maintaining a lock on it.
You can fix this error message by stopping the Firefox process manually. Open the task manager by right-clicking the task bar and clicking “Start Task Manager," click the “Processes" tab, click “firefox.exe" in the list, click the “End Process" button at the bottom of the list of processes and click the “End Process" button again. Repeat this process for each “firefox.exe" process you see in the list of tasks.
If Firefox doesn’t start normally after you do this, it’s possible the lock file is still in place in your profile. Click “Start," type “%APPDATA%\Mozilla\Firefox\Profiles\" into the search box in the start menu, press “Enter," double-click the profile with “default" in its name, look for a file named “parent.lock," right-click it and delete it.
The Program Must Close to Allow a Previous Installation Attempt to Complete
This error message is caused by a leftover “xpicleanup.dat" file that Firefox didn’t delete properly. You can delete this file yourself.
- Click “Start," type “%APPDATA%" into the search box in the start menu and press “Enter."
- Navigate to the “Local\VirtualStore\Program Files\Mozilla Firefox" folder in the “Application Data" folder.
- Delete the “xpicleanup.dat" file by right-clicking it and clicking “Delete."
- Restart Firefox.
Unable to Connect or Server Not Found
This Firefox error message usually indicates a problem with your Internet connection. Make sure your computer is connected to the Internet and that other programs can access the Internet. If you’re connected to the Internet wirelessly, verify you have a strong signal and connection to the nearest wireless network.
This error can have other causes relating to proxy, IPv6 and DNS prefetching if Firefox can’t connect, but other programs on your system can. Mozilla has detailed information about fixing these uncommon problems here.
This Connection is Untrusted
The website is using an SSL security certificate with a problem. If you click “Technical Details" on the error message page, you can see the exact problem.
One common SSL error message is “(website) uses an invalid security certificate. The certificate is only valid for (website). (Error code: ssl_error_bad_cert_domain)." This error indicates that someone is trying to impersonate a valid website or that the website is mis-configured. If it says that “http://website.com" is using a certificate only valid for “https://website.com," try accessing the website through “https://website.com."
Another common SSL Firefox error message is “(website)uses an invalid security certificate. The certificate is not trusted because it is self-signed. (Error code: sec_error_untrusted_issuer)." This error message indicates that your connection with the website is encrypted, but doesn’t verify the identity of the website. If you see this error message when connecting to an internal server you know doesn’t have a verified identity, it might be OK. If you see this error message when connecting to a website such as “eBay.com" or your bank’s website, this is a problem. If you trust the website and don’t expect identity verification, you can click “I Understand the Risks."
The Connection Was Reset, Was Interrupted, or Has Timed Out
This error message usually indicates a problem with the website you’re trying to connect to. There’s not much you can do to fix this error.
If you can connect to the website normally in other Web browsers, try clearing your cache.
- Click “Tools" at the top of the Firefox window and click “Clear Recent History."
- Click the “Time Range to Clear" box and click “Everything."
- Click “Details."
- Enable the “Cookies" and “Cache" checkboxes by clicking them.
- Click “Clear Now."
With a proxy error, Firefox is trying to connect to the Internet through a proxy server which isn’t available. You can fix this Firefox error message by telling Firefox not to use the proxy or correct your proxy settings.
- Click “Tools" and “Options."
- Click the “Advanced" icon.
- Click the “Network" tab.
- Click “Settings" under “Connection."
- Modify the proxy settings in the window. If you don’t want to use a proxy, click “No Proxy."
- Click “OK" twice.
Mozilla: Firefox Will Not Start - http://support.mozilla.com/en-US/kb/firefox%20will%20not%20start
Mozilla: Firefox is Already Running but is Not Responding - http://support.mozilla.com/en-US/kb/Firefox%20is%20already%20running%20but%20is%20not%20responding
Mozilla: The Program Must Close to Allow a Previous Installation Attempt to Complete - http://support.mozilla.com/en-US/kb/The%20program%20must%20close%20to%20allow%20a%20previous%20installation%20attempt%20to%20complete
Mozilla: Profiles - http://support.mozilla.com/en-US/kb/Profiles#How_to_find_your_profile
Mozilla: This Connection is Untrusted - http://support.mozilla.com/en-US/kb/This%20connection%20is%20untrusted
Mozilla: Error Loading Web Sites - http://support.mozilla.com/en-US/kb/Error%20loading%20web%20sites
Screenshots courtesy of author courtesy of Mozilla Firefox.