There are a few available based on FF, such as LibreWolf, and something based on Chromium, such as Brave, for example.
The problem is not their lack of existence, but their complete dependence on the source. No project is likely to be able to sustain itself and develop without the source. The main changes and fixes are created by the source, and everything else is just more or less configuration changes.
For example, if FF were to stop developing tomorrow, none of these modifications would have the resources imho to effectively develop and maintain the code. They could still exist, of course, but the scale of growing problems and bugs, especially security, will very quickly make such a modification very dangerous.
Browsers have become so large and complex that almost no one has the resources to effectively create a truly independent fork from the source. Even Brave, which is probably the most popular chromium mod, is fully dependent on it and each new version.
Nothing else that is not FF or Chromium that is relatively current in 2025 exists. There were and probably are some attempts to build on different engines, but I would not call it the final desired solution.
And projects like Ladybird are just emerging and at the moment are far, far away from the starting line. Building a completely new independent browser is a gigantic job and as a result, we currently have the world of FF and Chromium and various evolutions of this code under different names.
Of course, there is Edge, Opera, Chrome which is a bit different from Chromium, but it is still, in a big simplification, one and the same at the base, deriving from the Chromium code.
There is no point in writing about Safari because it is an Apple ecosystem. 