A. First go to 'File/Save For The Web'
B. Within the Save for the Web Panel, lower right, select the Browser Menu command option.
(Browser Menu command option has your default Browser icon next to it and is just to the left of the 'Edit in ImageReady' command button)
C. From the Browser Menu context list there are two additional options entitled 'Other...' & 'Edit List...'.

Select 'Other...' allows you to manually select the main Browser executable of your choice.
Select 'Edit List...' reveals all available browsers on your system including the (default) one used for the 'Preview in'.
If you installed a new Browser and it does not appear in the list hit the 'Find All' command.
If your choice of Browser does not appear, hit the 'Add' to manually to locate it on your system.
When your Browser choice is in the list, select it, then hit the 'Set As Default' command.