This addon mostly works fine, but it requires the "access-your-data-for-all-websites" permission.
At that point, I prefer to use "SingleFile", which also requires it, but it is recommended (and therefore I hope checked) by Firefox.

For details on the dangerous permission, see:
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https://support.mozilla.org/en-US/kb/permission-request-messages-firefox-extensions#w_access-your-data-for-all-websites

The extension can read the content of any web page you visit as well as data you enter into those web pages, such as usernames and passwords.