Add a fixed default-profile option to the new Firefox profile manager

[mokazemi]

The new Firefox profile manager currently allows Firefox to either display the profile chooser at startup or automatically open the most recently used profile.

Please add an option to designate one profile as the fixed default profile.

A user should be able to mark one profile - for example, `Daily` - as the default. Firefox should then open that profile whenever it is launched normally. Other profiles should remain available through the profile switcher or separate shortcuts, and opening another profile should not change the designated default profile.

This would be different from simply opening the last-used profile. The user should be able to select a persistent default profile that remains unchanged until explicitly changed.

My use case is having separate `Daily`, `Development`, and `Streaming` profiles. I want Firefox to open `Daily` by default, while opening the other profiles only when I explicitly select them. The current alternatives are either reopening whichever profile was used last or displaying the profile chooser every time Firefox starts.

The legacy Profile Manager provided equivalent default-profile behavior. Adding this option to the new profile manager would make it more suitable for users who maintain separate profiles but want one profile to remain the normal default.

Suggested behavior:

- Normal Firefox launch → open the selected default profile.
- Manual profile selection → open another profile when requested.
- Opening another profile → do not change the designated default profile.
- Profile chooser → remain optional and independent of the default-profile setting.

[kelimuttu]

Thanks for submitting an idea to the Mozilla Connect community! Your idea is now open to votes (aka kudos) and comments.