Shortkeys "do nothing" functionality persisting after uninstalling the extension

[ASong5]

I had keybinds configured in the vimium extension so that I can use shift+up and shift+down to scroll up and down a page. This didn't work properly because the default behavior of shift+up/down in firefox was to select text, which interfered with the vimium keybinds (it would select text instead of scrolling). So I installed the "shortkeys" extension to disable the native behavior of shift+up/down and everything worked as intended. But then I shortly uninstalled shortkeys out of curiosity's sake, and the original problem was still fixed, i.e., the selecting text behavior wasn't overriding the behavior I had set in vimium. I disabled vimium to test, and the default text selection behavior of firefox returned. So it looks like now vimium overrides the default behavior (which is what I wanted in the first place anyway). It seems like a very hacky solution that emerged out of curiosity, but I'm curious why this worked. Due to my lack of knowledge in the domain of browsers/web development, I am deciding to write a post here to see if anyone could share some light as to what may have happened.

PS: I know that I could have just mapped shift+up/down in shortkeys directly. I just didn't know of shortkeys until very recently (due to trying to figure out a solution to the original problem).

Thanks if you read this.