(Note: Xfire In-Game feature does not work. This is only for Playtime tracking.)

Step 1: Modifying the "xfire_games.ini"

Make sure to fully close out of Xfire.

Go to C:\ProgramData\Xfire directory.
Find the file "xfire_games.ini" and open it in Notepad.

Press Ctrl + F and find "roblox"

You should see something this:

[5718]
LongName=Roblox
ShortName=roblox
LauncherDirDefault=%localappdata%\Roblox\Versions\version-*\
LauncherUrl=http://www.roblox.com/
DetectExe=RobloxPlayer.exe
Launch=%UA_LAUNCHER_URL%
InGameRenderer= D3D9
InGameFlags=USE_PRESENT|ENABLE_MOUSE|USE_DINPUT_MOUSE|NEW_INPUT_SYSTEM
DetectServer=False

Step 2: Making Xfire detect Roblox

Swap that out for this.

[5718]
LongName=Roblox
ShortName=roblox
LauncherDirDefault=%localappdata%\Roblox\Versions\version-*\
LauncherUrl=http://www.roblox.com/
DetectExe=RobloxPlayerBeta.exe
ProcessWindowTitle=Roblox
PlayingDetectType=ByWindow
Launch=%UA_LAUNCHER_URL%
DetectServer=False

If you dont use Roblox's bootstrapper

Swap "Roblox" in "LauncherDirDefault" to the bootstrapper you use

For example: "LauncherDirDefault=%localappdata%\Fishstrap\Versions\version-*\"

Make sure to save the changes!!!

Step 3: Open Xfire

Start up Xfire and wait for it to finish scanning installed games on your PC.

Once that's done Xfire should be able to detect and track playtime for Roblox.