Yes, in that it doesn’t intterupt or disturb SqueezeCenter.
No, in that the SqueezeBoxes are not able to connect to a firefly share.
Squeezeboxes work only with SqueezeCenter, which is also open source. It requires a lot of memory and can be quite a CPU hog at times, so it’s not for the small NASes like firefly is.
I recently got a SqueezeBox (yes, I went to the dark side ;)) and have to say the device itself is very nice. On the other hand my old SoundBridge M1001 works with SqueezeCenter! It doesn’t work as fast as with firefly, but I’m able to control it through the webinterface of SqueezeCenter which is quite nice.
So if you plan on getting a SqueezeBox, make yourself aqcuinted with SqueezeCenter first. You don’t need a SqueezeBox for that.