I have recently discovered Palemoon which is a fork of Firefox.
Apparently the Palemoon creators didn't like all the bloat and
nonsensical ideas in firefox so completely reworked the code
and rebuilt from source.
Now to the relevance to BBLean/Blackbox.
I have found that Palemoon is more easily themed by BBLean mainly
because it doesnt use Australis and is more streamlined. It is well worth trying
but I would suggest that you use the 32 bit version even on 64 bit windows.
I am certain people who have defected from Firefox to other browsers will
love how it responds to BBLean and patched windows themes, as I do.
Well worth a try if you love Firefox but hate how it has developed over the last 18 months