This is Firefox's specific error message for the same underlying issue as ERR_SSL_VERSION_OR_CIPHER_MISMATCH in Chrome — no shared cipher suite between the client and server.
The fix
Check your server's supported cipher suites and TLS versions against what the connecting client actually supports. If you've recently hardened your TLS configuration to remove legacy ciphers, this is often an expected trade-off for older clients — confirm the affected client is genuinely one you need to support before loosening your configuration.
What causes a complete absence of overlapping cipher suites specifically
This more extreme version of a version/cipher mismatch typically occurs when a server has been configured with an unusually restrictive, modern-only cipher suite list while the connecting client is running genuinely outdated software with no overlapping support at all — a gap wide enough that no negotiation is even possible.
How to balance security hardening against this specific compatibility risk
Using Mozilla's intermediate compatibility configuration preset, rather than the strictest modern-only option, generally avoids a complete overlap failure while still maintaining strong security — reserving the most restrictive configuration for situations where you have specific evidence every client in your audience supports it.
What to check first if you're the server administrator facing this error
Reviewing your server's currently configured cipher suite list against what a reasonable, current compatibility standard (Mozilla's intermediate preset, for instance) actually requires often reveals an overly restrictive configuration that's excluding more legitimate clients than intended.
How this differs from the more general SSL version/cipher mismatch error covered elsewhere
This error represents the more extreme, complete-failure version of a cipher mismatch — where a version/cipher mismatch might mean partial overlap with a suboptimal but workable result, this error means literally no shared option exists at all between client and server.
A final note on documenting your intended compatibility target
Documenting your server's specific compatibility target (modern, intermediate, or old, following Mozilla's standard tiers) makes future troubleshooting of this exact error considerably faster, since you'll immediately know whether a reported failure falls within your intentionally supported range or represents a genuinely unexpected gap.
What specific server hardening changes most commonly trigger this complete overlap failure
Disabling every legacy protocol version and cipher suite simultaneously, rather than a more gradual, staged hardening approach, is the most common trigger — a genuinely ancient client with no modern support at all can end up with zero overlapping options against an aggressively hardened, modern-only server configuration.
How to choose an appropriately balanced configuration rather than the most extreme option available
Mozilla's intermediate compatibility tier represents a deliberately chosen, well-reasoned balance between security and broad compatibility — reserving the most restrictive modern-only tier specifically for situations where you have concrete evidence every client in your actual audience supports it avoids inadvertently locking out legitimate traffic.
A quick closing checklist
A quick closing checklist covers reviewing your server's current cipher suite configuration against a reasonable compatibility standard, checking real visitor analytics before hardening further, and documenting your chosen compatibility target for future reference.
How this error's complete failure differs meaningfully from a partial compatibility issue
A partial mismatch still allows some clients through while excluding others; this complete overlap failure means literally zero shared configuration exists between the specific client and server involved — a more extreme, less common scenario than the general version/cipher mismatch covered elsewhere.