If you want to make Firefox the default browser for all users, install Firefox with/as an administrator.
See
Creating a new default profile - MozillaZine Knowledge Base for information on setting up a default profile
and
Profile folder - Firefox - MozillaZine Knowledge Base
and especially this one:
https://support.mozilla.org/en-US/questions/901549 .
Once Firefox is installed, open a Windows 7 Run box and go to %APPDATA% then Mozilla, then Firefox and finally to Profiles directory. There will be another directory with a name like: 19gx53wq.default This is your current profile as the current user that installed Firefox. Copy that folder to
C:\Program Files (x86)\Mozilla Firefox\defaults\profile\
Be careful that the profile you are copying does not have passwords stored for web sites. Doing this will copy the bookmarks that are stored for the user that you are using for the master default.
The Mozilla support page at
https://support.mozilla.org/en-US/questions/901549 says:
" Did you create a defaults\profile in the Firefox program folder (C:\Program Files\Mozilla Firefox\)?
All files in that folder will be moved to each newly created profile folder.
For giving prefs a default value it is better to do that via a mozilla.cfg file.
Use a mozilla.cfg file in the Firefox program folder to lock prefs or specify default values.
Place a file local-settings.js in the defaults\pref folder where you also find the file channel-prefs.js to specify using mozilla.cfg.
pref("general.config.filename", "mozilla.cfg"); pref("general.config.obscure_value", 0); // use this to disable the byte-shift See:
You can use these functions in mozilla.cfg:
defaultPref(); // set new default value pref(); // set pref, but allow changes in current session lockPref(); // lock pref, disallow changes