Notes:
Tested on XP SP2 (WB version will not matter).
Only a small amount of testing done, but appears to work ok.
It is slower to load, but thats a minor complaint.
Oh and it's your decision

Here we go..
1) Close WeBuilder.
2) Remove the Mozilla ActiveX Controller V1.7 (use add and remove programs).
3) save this http://releases.mozilla.org/pub/mozilla ... .win32.zip
4) Extract the zip to some folder, eg: C:\MOZZIE (Take your pick).
5) Now use regsvr32 C:\MOZZIE\MOZCTLX.DLL (start run will do).
Now start WeBuilder, your preview pane with Firefox will now use the latest gecko engine.
Add this to a test html file: <button onclick="alert(navigator.userAgent)">Version</button> then preview, welcome to 1.9.x

UNINSTALLING - Just in case

Just use start run: regsvr32 /u C:\MOZZIE\MOZCTLX.DLL
You should get a message stating successful unregistration.
Then reinstall the old version.
Thats it, now xulrunner the thing you downloaded is upgraded often, just check http://releases.mozilla.org/pub/mozilla ... r/releases for more goodness.