Long answer:
My default browser these days is SRware Iron (it's a variant of Chrome that doesn't pass all my browsing data to Google but still supports all the Chrome extensions), but I also use Chrome, Firefox, Safari on my MBP, and for work I'll even dabble in IE to make use of some custom ActiveX controls. On my G5 Mac I use TenFourfox, a variant of recent Firefox compiled for the PowerPC architecture Macs. I also have Opera installed, but I tend not to use it much.
Short answer:
I'll use any browser for BM that will allow the easiest altering of the default color scheme. Which one do you suggest?