Firefox Debugger instructions (#71938)
### What?
Added Firefox debugging instructions to Next.js docs
### Why?
To support developers who use Firefox for debugging
### How?
- Added Firefox config to VS Code launch settings
- Added Firefox-specific debugging instructions
- Updated relevant docs sections to include Firefox alongside Chrome
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Co-authored-by: Novalinium <nova@noblejury.com>
Co-authored-by: Delba de Oliveira <32464864+delbaoliveira@users.noreply.github.com>
