How to Join a string array?

[|2eM!x]

In vb6 we could simply do Join(sourcearray, delimiter)

But I cant find the equivalent in vb.net, Is there?
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Also, I'm writing a FireFox proxy changer, and Im looking for input on how I could improve my code:

Option Strict On
Option Explicit On
 
Public Class Form1
 
    Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
        Dim Path As String = String.Empty
        Dim js_Prefs() As String
        Dim RndProxy As New Random
        Dim Proxies() As String = (My.Computer.FileSystem.ReadAllText(Application.StartupPath & "\Proxies.txt")).Split(CChar(Environment.NewLine))
        For Each FxUserName As String In IO.Directory.GetDirectories("C:\Documents and Settings\" & Environment.UserName & "\Application Data\Mozilla\Firefox\Profiles\")
            Path = FxUserName
        Next
        js_Prefs = My.Computer.FileSystem.ReadAllText(Path & "\prefs.js").Split(CChar(Environment.NewLine))
        Dim IP As String = Proxies(RndProxy.Next(0, Proxies.GetUpperBound(0))).Trim
        For I As Int32 = 0 To js_Prefs.GetUpperBound(0)
            If js_Prefs(I).Contains("network.proxy.http_port") Then
                js_Prefs(I) = js_Prefs(I).Substring(1, js_Prefs(I).LastIndexOf(",") + 1)
                js_Prefs(I) += IP.Substring(IP.IndexOf(":") + 1) & ");"
                Exit For
            ElseIf js_Prefs(I).Contains("network.proxy.http") Then
                js_Prefs(I) = js_Prefs(I).Substring(1, js_Prefs(I).IndexOf(",") + 1)
                js_Prefs(I) += Convert.ToChar(34) & IP.Substring(1, IP.IndexOf(":") - 1) & Convert.ToChar(34) & ");"
            End If
        Next
    End Sub
 
End Class