I am having trouble with CSS in Firebug

Hello,
I posted this here because I had no where else to go. You all are experts.
OK, here is the deal.
1. I add a custom CSS rule to someones page, let's use this.

#logo {
display: none !important;
}

2. I minimize firebug or switch to a different tab, everything disappears.
3. Everything disappears from the CSS code, but the CSS changes still stay in effect on the website.

Anybody have any ideas.
Thanks.