What it means
Firefox can block known dangerous or potentially unwanted downloads and can warn when software is uncommon. A new release may be rare without being malicious; a common file can still be harmful if a trusted source is compromised. The decision should combine warning category, publisher and download route.
A real-world example
An installer from a search ad is flagged as uncommon. The user finds the publisher's actual site has a different filename and release version, so discards the ad's file.
What to do
Read the Downloads-panel explanation and independently locate the publisher. Keep browser protection enabled while investigating; obtain a new copy through a verified route if needed.
The distinction that matters
A signature or matching vendor hash establishes a narrower fact about origin or byte-for-byte identity. It does not promise the app is safe or appropriate for your task. An antivirus clean result similarly cannot erase a suspicious provenance trail.
Mozilla's warning categories
