This routine replaces a string in the text file user.js.  The file is located in each user’s profile which also contains a random string as part of the path name.

Imports System.Net
Imports System.IO

Public Class Form1

Private Sub Form1_Load(sender As System.Object, e As System.EventArgs) Handles MyBase.Load
 Dim strPath As String = "c:\users"
 Dim arrFolders() As String = System.IO.Directory.GetDirectories(strPath)
 Dim MozPath As String

'----------------------------------------------------------------
 ' get list of profiles
 '----------------------------------------------------------------
 For Each arrFolder As String In arrFolders
 MozPath = arrFolder & "\AppData\Roaming\Mozilla\Firefox\Profiles\"

If My.Computer.FileSystem.DirectoryExists(MozPath) = True Then
 Dim arrGUIDpath() As String = System.IO.Directory.GetDirectories(MozPath)

For Each arrGUIDfolder As String In arrGUIDpath
 If My.Computer.FileSystem.FileExists(arrGUIDfolder & "\user.js") = True Then
 Console.WriteLine("notepad " & arrGUIDfolder & "\user.js")

'----------------------------------------------------------------
 ' 1. rename the user.js file
 ' 2. replace the proxy defintion string tinto user.js
 '----------------------------------------------------------------
 Try
 My.Computer.FileSystem.DeleteFile(arrGUIDfolder & "\user2.js")
 Catch ex As Exception
 Application.DoEvents()
 End Try

My.Computer.FileSystem.CopyFile(arrGUIDfolder & "\user.js", arrGUIDfolder & "\user2.js")

My.Computer.FileSystem.WriteAllText(arrGUIDfolder & "\user.js", _
 My.Computer.FileSystem.ReadAllText(arrGUIDfolder & "\user2.js").Replace("user_pref(""network.proxy.type"", 4);", _
 "user_pref(""network.proxy.type"", 0);"), False)
 End If

'----------------------------------------------------------------
 ' clean up to avoid confusion in the future
 '----------------------------------------------------------------
 Application.DoEvents()

Try
 My.Computer.FileSystem.DeleteFile(arrGUIDfolder & "\user2.js")
 Catch ex As Exception
 Application.DoEvents()
 End Try
 Next
 End If

Next
 End
 End Sub

End Class