In previous versions we were able to bring up the find dialog in custmon (NO editboxed) windows by simply pressing 'f', in 6.1 this doesn't work anymore
is it removed or bugged?

In mIRC 6.03:
/window test
f

Nothing happens.

I think he meant ctrl+f or alt+f (I always forget which). I'd test myself but I'm at school and no mIRC here

//window @test | echo @test pokemon
ctrl+f

^works fine on 6.1.

I ment what I said, I'm pretty sure I used to do this by presing only f, maybe it's just my keyboard but I doubt that
I'll get some older versions and see if I'm in a delusion

well I'm right
if anyone of you has 6.03 please try that:
/window @a
/echo @a a
activate @a and press "f"
it works for me, on nt4

(there must be something in the window otherwise neither "f" neither "ctrl+f" works)

Indeed, it works here in 6.03 as well, but not in 6.1, I'd say it's a bug.