Forked software is usually reasonable, but for me, the sheer complexity of the graphical, interactive web sadly begets an exception for modern browsers. Even if much of the work is offloaded to the corporations backing modern free/libre browsers (e.g. keeping up with new web standards and technologies), it certainly goes without saying that rebasing with upstream or even just applying the latest security fixes is no trivial task I would entrust any browser fork with minimal developer backing with.

This problem is especially worsened with the kind of user base that firefox forks tend to attract, from what I have seen. These users tend to ask very high-level questions e.g. "is waterfox more secure than pale moon" and will usually blindly switch from one fork to the next based on poorly-backed, unsubstantial crowd opinion. No userbase means waning support + maintenance. If you ask me, I think in the very special case of browser technologies, it would be more beneficial if the developers and users of firefox forks directed their energies towards making generally desirable changes in upstream.