I do not understand why someone w/ a green card needs to be afraid of being fired. Isn't the whole point of it to firm up your status as someone who is allowed to stay on indefinitely with very few conditions, one of them being % of year spent in the US, plus not committing certain serious offences (felonies?)
That is true. The parent probably meant to say Visa not Green Card. I can definitely relate to the feeling of being "stuck" in a bad situation while on a Visa and/or while waiting for a green card to be processed. It's such a huge relief when you finally get that freedom to move around in the job market.
I am sure he meant people on a work visa, whom the employer had applied for and are waiting to get a green card. The process can take from 2 to 10 years, so you are at the mercy of the employer during that time- where abuse often worsens because of the power leverage.