Open the favorites folder
Open Tinderbox's Help menu and click the item Reveal Support Folder in Finder (pre v5.10.2 this was located further up the menu):
View the favorites folder
Tinderbox's support folder is revealed in Finder. Now, select favorites (note the US English spelling):
Close Tinderbox whilst adding new files.
To ensure the changes you make are detected, close Tinderbox at this point.
[Actually, closing/re-opening Tinderbox is not strictly necessary but can help in some circumstances to ensure the 'Open Recent' menu is properly refreshed.]
Add the 'starter' file
Copy/paste or drag-drop your 'starter.tbx' file into the favorites folder. Note that you can also place aliases to TBXs in this folder. By doing that the original TBX stays in its current location, but can now be opened via Tinderbox's 'favorites' (File menu, Open Recent sub-menu, below the rule).
Set File Info
In Finder, select your 'starter' TBX and use shortcut Cmd+i to open Finder's Get Info dialog for that file. In the General section of the dialog tick the Stationery pad box.
Re-open Tinderbox and select the starter
Re-open the Tinderbox application. Choose the File menu, Open Recent sub-menu. Notice 'starter' is now listed? Click that entry...
Open a new file
Tinderbox now opens a new, unsaved TBX, based on the starter.tbx file. The actual source TBX file remains unaltered.
Edit the the new file as required and save it. You may also want to review the new TBX's document-level preferences ( [Cmd]+ ), e.g. for auto-save and the like.