Unfortunately, auto-checking of extensions is only impemented for Chrome extensions - not Firefox. To download a Firefox extension it has to be explicitly whitelisted by sending a request to Netfree.
@david-k I don't know if this is still relevant but you can change the language after installing Firefox in the setting of the browser.
Search for "about:preferences" without the quotes.
Scroll down until הגדרות שפה and choose English from the dropdown menu
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