Since the last update of my Firefox Developper Edition, the script is now working.
In the changelog they specify that was a problem with script execution on CSP secured pages :
Fix script execution at CSP secured pages
Fix script execution at service worker cached pages
Make GM_cookie/GM_xhr always use the source tab's cookie store ID
Fix GM_info.isIncognito by showing a per tab value
Fix GM_download of blob objectURLs