Hello,
It is a little problem, but I just don't understand.
When I run firefox (a new window), it can have 2 differents looks. A grey one, obviously not 'integrated', and a blue one that I prefer and think is supposed to be the firefox integration I heard about.
When I launch firefox from a launcher or via krunner, it is blue (good).
When no 'blue instance' is running and I run it from the terminal, I get the grey one. But if there is already a blue instance running, it will be blue.

The problem is that most of times, I run it via the command line (a xbindkeys shortcut, actually). So I end up with the ugly grey thing .

When I look at the launcher, I see nothing that indicates a different command or option.

Is there a way to launch it via command-line using a launcher or krunner?

Have you compared the environment variables present in your shell, and those present in the environment of KRunner, etc? Your shell may be clearing needed variables. (Most probably, one named GTK_RC_FILES or GTK2_RC_FILES)

Thanks, it works now.
Is there another way to know the env variables in krunner than running env >output.txt? In other words, what are stdout and stderr in krunner? Is it redirected to /dev/null by default?

The stdout and stderr channels are likely redirected into ~/.xsession-errors. This is dependent upon how KRunner is started however - which is normally from kdeinit4 - which itself is started by the startkde script (through ksmserver).