Is there a way to automatically make CSS/HTML changes to a web page each time it is loaded?

I sometimes use Firebug to fix problems with w开发者_StackOverflow社区ebpages or to hide certain things. I am simply a user of these webpages and cannot change the source. I would like to be able to automate this so that each time I visit a website, certain modifications are performed. Right now, I need to redo the same edits each time the page is loaded.

I am open to using other tools than Firebug, since I'm not sure it is capable of doing this.

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I have successfully used Greasemonkey for this task.