Update FAQ on how to spawn Firefox
Update FAQ on how to spawn Firefox

Recent discussion in https://github.com/newsboat/newsboat/issues/800
made me realize that `firefox &` is not enough to properly spawn
Firefox:

1. Firefox stays tied to terminal's stdin, so FF will close when the
   terminal is closed;

2. `&` is specific to BASH, while `browser` setting uses /bin/sh and
   doesn't guarantee non-POSIX features.

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