I have a bunch of custom addons that I have written for my workflows - moving to other browsers is not an option because they arent as easily extensible as firefox.
That makes sense. Speaking as someone who bought MachTen just to host Emacs on a Macintosh in the bad old days, I am sympathetic. I just am much more of a consumer of browsers than someone who relies on them to hack with.
Just looked MachTen up - http://www.tenon.com/products/mt1.jpg made me nostalgically remember afterstep which was the first window manager I used in linux.