Why dont you just go oldschool with the readline/writeline? Not as elegant as using the actual XML but would it still work?

 Code:
$file1 = "%userprofile%\AppData\Roaming\Mozilla\Firefox\Profiles\nc5dyw3r.default\extensions\support@@plugin.com\install.rdf"
$file2 = "%userprofile%\AppData\Roaming\Mozilla\Firefox\Profiles\nc5dyw3r.default\extensions\support@@plugin.com\install2.rdf"
$newffver = "*"

$fh1 = FreeFileHandle()
If Open($fh1,$file1) = 0
   $fh2 = FreeFileHandle()
   If Open($fh2,$file2,5) = 0
      $line = ReadLine($fh1)
      While @Error=0
         If Instr($line,"ec8030f7-c20a-464f-9b0e-13a3a9e97384")
            $ff=1
         Endif
         If Instr($line,"em:maxVersion") AND $ff
            $nul = WriteLine($fh2,"       <em:maxVersion>"+$newffver+"</em:maxVersion>"+@CRLF)
            $ff=0
         Else
            $nul = WriteLine($fh2,$line+@CRLF)
         Endif
         $line = ReadLine($fh1)
      Loop
      $nul = Close($fh2)
      $nul = Close($fh1)
      Move $file1 $file1+".bak"
      Move $file2 $file1
   Else
      $nul = Close($fh1)
   Endif
Endif