Mozilla Firefox errors and troubleshooting

Troubleshoot Firefox display, profile and certificate problems. Use Troubleshoot Mode to narrow down the cause before refreshing browser settings.

Before choosing a guide

◫Mozilla FirefoxTroubleshooting guide

Firefox crashes

Firefox Desktop (general Mozilla support procedure; no specific version)

Checked 2026.09.05Read guide ↗

◫Mozilla Firefox DesktopTroubleshooting guide

Firefox problems continue after isolating extensions and settings

Firefox Desktop; available in Firefox 150 or later when using the new profile management

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◫Mozilla FirefoxTroubleshooting guide

A certificate issuer is unknown

Firefox DesktopSEC_ERROR_UNKNOWN_ISSUER

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◫Mozilla FirefoxTroubleshooting guide

Firefox displays Server Not Found

Firefox for desktop on Windows, with another browser available for comparisonServer Not Found

Checked 2026.09.12Read guide ↗

◫Mozilla FirefoxTroubleshooting guide

Firefox says it is already running after exit

Firefox DesktopFirefox is already running but is not responding

Checked 2026.09.05Read guide ↗