[zzo38computer]

You might want some of the new features (such as TLS 1.3, WebP, some security fixes, etc), but avoid some others (such as HSTS, many new web APIs, secure contexts, AI, some CSS commands, etc), and want to keep some features that are removed in later versions (such as several settings, and many other things).

[hekkle]

You're right, you may want some features but not all of them. That is why firefox provides the flags for you to turn features on/off. You mention that a user might wast "TLS 1.3, WebP, some security fixes, etc". I would argue that if a user knows what these are, they are capable of working out a flag.

[zzo38computer]

Not all of the functions can be controlled by the flags, though.