How to hide an elemet on a Webpage with CSP

I want to hide an element on a webpage permanently (every time I load the site the element should be hidden) but can't do so because of CSP. I first wanted to do something with javascript and to block CSP with an addon. This didn't work, I still get "inline script blocked". I also tried to disable CSP overall in Firefox but got the same result. Doing it with CSS also didn't work because it is also gets blocked by CSP. I found some workaround by blocking the CSP Header but since I'm not very familiar with this I didn't want to dive in before asking here because I think there should be some simple solution since I just want to do the same as I would by right-clicking and pressing "Inspect Element" and the just hiding the element. Is there anyway to do so? I've been stuck for two days. Please help. :)

Sorry for my English I'm not a native speaker.