Yet another CORS issue

I’m developing this little extension :
Recently I added 2 more dictionaries to fetch() data from. I added the urls in my manifest.json file under “permissions” section but can’t get the data due to CORS restrictions I can’t explain : It’s fine with all the other dictionaries, only the 2 new ones are not okays. I can’t explain myself what I’m missing. I’m testing by loading unpacked extension or xpi file, it’s the same issue. I’ve deleted the profile and reinstalled firefox because i suspected my testing process for side-effects due to repeated removes/adds but it’s still the same.
I desperatly published the new version anyway in case there’s an ID problem that would be solved by the publishing process.
I’m developping both for Chrome and Mozilla. It’s fine with Chrome-like browsers.