Go to 'System Preferences -> General' - browser selection if half way down the dialog box - see attachment below (I just have Chrome installed as a alternate) - let us know if this solves your question. Dave
Well, then you need to tell us which OS X you are using - that is the current option in Yosemite, so if using an earlier OS then you may have to go into the 'Preferences' menus of the actual browser - some information HERE - as an example, when I open the Chrome browser, then enter Preferences, the option is given to make it my default (at the bottom of the attached image) - I don't have Firefox on this computer, but would assume a similar feature is present. Dave
I have both Safari and Firefox browsers installed under Mavericks,
Safari -> Preferences -> General tab has Default Web Browser as a drop-down option.
Firefox -> Preferences -> General has ‘Make Firefox your default browser’ option button.