Well yes, but it’s the patches that make them special. Every Firefox fork that disables Mozilla PPA by default is another browser that cuts into Mozilla’s attempt to resell private data to advertisers while marketing it as private (which is, I kid you not, a reason they say they needed it enabled by default).
And considering Firefox itself is still open source, it’s a completely valid browser to base a fork off of. Especially when the only serviceable alternative is Chrome right now.
someone else can probably give a more comprehensive/correct answer but here is how I understand it. i believe chromium is open source and chrome is mostly chromium but also some proprietary (and therefor unknown) bits are included. whereas firefox is entirely open source, meaning you could compile it yourself and still end up with the same package.
