webgui: use fork() for starting firefox in headless mode
webgui: use fork() for starting firefox in headless mode

To use firefox in batch mode at the same time as other firefox is
running, one should use extra profile. This profile should be created
first:

   firefox -no-remote -CreateProfile root_batch

And then in the start command one should add:

   $prog -headless -no-remote -P root_batch $url

By default, no profile is specified, but this requires that no firefox
is running

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