firebug edited code disappears

Forgot to mention I’m using TwentyTwelve theme.

Firebug is only changing your code in the browser itself. It’s useful for trying CSS changes out without committing to them and for troubleshooting, but You need to actually edit the CSS to make that work.

The proper way to accomplish this is to create a child theme for TwentyTwelve (which in your case would just be a folder and a CSS file) and then override the CSS there:

https://codex.www.ads-software.com/Child_Themes

Thanks greendemiurge.