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FicLab browser extension is also very good for both completed stories and for getting ongoing stories into Calibre so that FFF can be used to update them.
Like the WebToEpub method, the information it downloads is available to FFF in the default browser web cache. Further, both WebToEpub and FicLab handle FFN rate limiting to avoid triggering Cloudflare. One advantage it has over WebToEpub is that it adds a blue download button for any and all fic links as they appear on any served FFN web pages as well as on the fic page itself.
Do note that it presently doesn't work on Chrome due to Google's Manifest v3 bullcrap, but still works well on Firefox-based browsers. The author told me a couple months ago that they are slowly working to update it to Manifest v3 (which is a major undertaking for essentially a hobby project) and also that, as part of the update, they intend that it will work on the new FFN website when it is eventually released.
You use it just like you would WebToEpub. Simply copy the FFN link for the fic, then click the FicLab download button for the fic on the webpage. When FicLab pops up, use FicLab to generate the epub. When the epub save box opens up, click the cancel button. Then, enter the FFN url link into FFF for download as you normally would. FFF will pick up the pages downloaded by FicLab directly from the browser cache of the default system browser.