@oblomov
Yes, it's a "hard fork" of Firefox.
Most of it is still Mozilla code from the days of Firefox 3 (Pale Moon started in 2009), but over the years and especially after Electrolysis (Firefox's multi-process architecture) things diverged so much they are completely different products.
Unfortunately people see "Firefox 3" and scream "old and insecure!!!" like it's a religious mantra, but the platform is developed to this day and you can use it as your primary browser just fine, like I do. In some cases it's even more secure than the rest: when there are issues with certificates or TLS you are warned instead of connecting at all costs, certain abusable javascript API are not implemented or intentionally crippled, etc.