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I'm pretty sure Pale Moon will continue to be developed. Its source is at Github. The main sole developer is Moon Child. You can find him at the Pale Moon forum.
The useragent add-on is called UAControl. In Pale Moon you can use the add-on Classic add-ons archive to retrieve many of the now defunct, but still useful XUL-based add-ons. Releases . JustOff/ca-archive . GitHub
Also, at the Pale moon website are some extensions you can use.
Another add-on that's a companion to UAControl is User-agent JS Fixer.