Changes to WebIDL files (ending in '.webidl') are being blocked by the IDL checking hook. WebIDL isn't XPIDL, and doesn't have UUIDs to change. I think there is a bug in this line from the script: if file.endswith('idl'): A file that ends in '.webidl' also ends in 'idl', which I'm guessing must be the problem. Presumably changing 'idl' to '.idl' would fix it, though I don't know if there are any other wacky endings you'd want to catch.
FWIW, this was mentioned in review of the patch. https://bugzilla.mozilla.org/show_bug.cgi?id=813809#c9
Created attachment 757970 [details] [diff] [review] Patch v1
https://hg.mozilla.org/hgcustom/hghooks/rev/e40e353145af No hurry to get this deployed in particular, I guess.