I had to switch back to v1.2.5. v1.3.0 is reloading the page twice and then using again the default language.

Strange user-interface.
Like in Brave (Chrome), the add-on only works on pages loaded via http(s), not on locally stored pages that you want to test under changed conditions prior upload.
But different from Brave, where just nothing happens, trying to switch the navigator-language on local files may seriously bedazzle Firefox... I saw a page continuously reloaded and languages switched without me even touching the mouse ... makes you dizzy.