I'm using Windows 10 and my default browser is Chrome. If I go to the Firefox directory (C:\Program Files (x86)\Mozilla Firefox) and open the command prompt and type:
ipython notebook --browser='firefox'
then the notebook opens as expected. However if I navigate to the folder which contains my files and try to do something like:
ipython notebook --browser='C:\Program/Files/(x86)\Mozilla/Firefox\firefox'
then it does not launch. The error is "No web browser found: could not locate runnable browser". I have tried many variations of this with no success. This was brought up in a similar thread a couple years ago (Launch IPython notebook with selected browser) with no resolution that I could see.