Strange behavioral quirk of vimium keybind.

I had keybinds configured in vimium 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.

Thanks if you read this.

Reproduce the reported sequence in Firefox with Vimium and the Shortkeys extension: configure Shift+Up/Down, disable the native behavior, uninstall Shortkeys, and compare the result with Vimium disabled. The issue names no file, test, or entry point, and provides no concrete change or completion condition beyond explaining the behavior.