I've used firebug in Firefox few times to alter the CSS of someone else's website - to make it more readable for myself in my own browser, by arranging the margins and making changes to elements other than just the text size. Is there a way I can permanently store this addition to the CSS for my own personal use, so that whenever I revisit the webpage, it will load the customized CSS styling in my browser?
Ideally, I would want to store only the additional CSS alterations I made, instead of essentially replacing what the website has with the version I modified - so that if they make changes, I don't end up keeping the old styling, and less of my customization breaks. I would also like the option of what URL filter I can use to decide which pages to optionally use my custom styling on.
I understand there might also be a solution where a browser script can be run which would insert a CSS rule into the webpage, but I have no idea how I would be able to accomplish that, so if that is an option I would like to hear more about how I could do that.