Snapshot builds of versions under development are normally done daily. They represent the latest state of the sources for each respective version. You can find them at the following locations:

Platform   Versions
All Firebird 6.0 Firebird 5.0    
Windows     Firebird 4.0 Firebird 3.0
Linux     Firebird 4.0 Firebird 3.0

Alternative locations

web.firebirdsql.org/downloads/snapshot_builds/

Alternatively, you may find recent snapshot builds at Appveyor:
Go to https://ci.appveyor.com/project/FirebirdSQL/firebird/history
Find and click the tag you are interested in

  • Click the Job Name
  • Select the Artifacts tab
  • Download the desired snapshot image file