Remove or add items to firefox open menu

I'm trying to customize my Firefox a little coming from chrome. I found that the 'open menu' is a very clutter. I'd like to remove some items thanks to the option Firefox gives us I believe with chrome.css

Is there a way I can see the css of the items like an inspector for the web?

I found this question to remove the entire menu, but I only want to remove some items, not the whole menu.