Does using the -install-global-extension option to install the British English dictionary via its .xpi make any difference? I've always installed extensions using this method and never had any problems. I use an AutoIt script to do this (attached to this post) which automates the installation of Firefox using its native installer.
Out of interest, is FFDeploy definitely compatible with Firefox v3.5 since it hasn't been updated in almost 5 years? I had a quick look through the CreateProfile.vbs and it doesn't seem to use Firefox -CreateProfile default to create the default profile. Not sure if this makes any difference though.