The only things you will lose, if you have them, when updating Firefox, are:
- Any extensions that are not compatible will be disabled until a new version is out (but if there is a new version, it will download it and re-enable).
- If you changed the Dock Icon for FF, you will have to copy it over again
- If you added search plugins to the search bar, you will either have to manually copy them out of the old FF directory and into the new one, or re-install them.
Of these, only the last, if it applies, is really a nuisance. If you look in the package contents, they're in firefox.app/contents/macos/searchplugins -- you can copy all of the contents of this folder to a new folder, and then put it back in once you put in the new FF. If you want to get fancy, you can re-locate them, and then put in a symbolic link, I guess.
I have no idea why these are in the app folder and not elsewhere. Perhaps it has something to do with executable code? In any event, it's the biggest upgrade nuisance with FF.