Close your Visual Studio and install the DDEX Provider for Visual Studio and the ADO.NET Data Provider, both can be downloaded at the following link:
https://www.firebirdsql.org/en/additional-downloads/
After the installation go to the Machine.config file located in %windir%\Microsoft.NET\Framework\[version]\config\machine.config
for 32 Bits or
%windir%\Microsoft.NET\Framework64\[version]\config\machine.config
to 64 Bits
And look for <DbProviderFactories>
you see that the DDEX installer inserted two lines into Firebird, removing one and leaving only one.
Here’s an example of how it should look:
<system.data>
<DbProviderFactories>
<add name="Microsoft SQL Server Compact Data Provider 4.0" invariant="System.Data.SqlServerCe.4.0" description=".NET Framework Data Provider for Microsoft SQL Server Compact" type="System.Data.SqlServerCe.SqlCeProviderFactory, System.Data.SqlServerCe, Version=4.0.0.0, Culture=neutral, PublicKeyToken=89845dcd8080cc91"/>
<add name="MySQL Data Provider" invariant="MySql.Data.MySqlClient" description=".Net Framework Data Provider for MySQL" type="MySql.Data.MySqlClient.MySqlClientFactory, MySql.Data, Version=6.9.7.0, Culture=neutral, PublicKeyToken=c5687fc88969c44d"/>
<add name="FirebirdClient Data Provider" invariant="FirebirdSql.Data.FirebirdClient" description=".NET Framework Data Provider for Firebird" type="FirebirdSql.Data.FirebirdClient.FirebirdClientFactory, FirebirdSql.Data.FirebirdClient, Version=5.7.0.0, Culture=neutral, PublicKeyToken=3750abcc3150b00c"/>
</DbProviderFactories>
</system.data>