There are a couple of options for keeping FRx around to connect to GP 2010 or GP 10:
Option 1
Install and run FRx from a 32-bit machine that has not had a GP 2015 client installed on it.
Option 2
If FRx is needed on a computer where GP 2015 has been installed, both GP 2015 and Dexterity14 will need to be removed. In some cases, all instances of GP and Dexterity need to be removed and then you can reinstall GP 2010/Dexterity11 or GP 10/Dexterity10. This should give you a GPConn.dll file of 11.X or 10.X in one or both of these folders:
C:\Program Files (x86)\Common Files\Microsoft Shared\Dexterity
C:\Program Files\Common Files\Microsoft Shared\Dexterity
You may need to manually register the correct 11.X or 10.X version of GPConn.dll with these steps:
1. Use Windows Explorer to find the file, right-click on it, choose Open With, and Choose Program.
2. In the Open With dialog box, click Browse, then browse to the Windows\System32 folder on drive C.
In the System32 folder, highlight the Regsvr32.exe file and click Open. In the System32 folder, highlight the Regsvr32.exe file and click Open.
4. Click OK and you should receive a message that the file registered successfully.