How can I save the "external" page edited in developer mode?

Question

css

Greetings!
The situation is the following. There is an external page with a graph (essentially an ordinary jpg that is updated every n seconds) and a bunch of other information that I don't need. Moreover, this graph-picture is almost twice as wide as the resolution of my monitor.
Using the Firefox developer mode, I added to the code ... the resolution I needed and removed other unnecessary elements - profit! =)
But, of course, when you restart the browser, all changes disappear.
I saved the modified code and can replace it every time through the same developer mode, but there is probably a smarter way/plugin that automates this process.
Can you suggest how to do this? And then I'm more of a hardware, a beginner in programming.

Answer 1

Your question is solved with the Stylish plugin for the browser. The essence of his work is that for any page you can apply custom CSS.

Answer 2

hmm maybe https://addons.mozilla.org/en/firefox/addon/stylish/ ?