I use Yandex Browser [Chromium-based] so, unfortunately, FireMonkey isn't an option. The article you linked to is quite vague as to what the security implicati

I use Yandex Browser [Chromium-based] so, unfortunately, FireMonkey isn't an option. The article you linked to is quite vague as to what the security implications might be. It says that FireMonkey extensions run in a sandbox. But you're still being asked the usual "This extension wants to see and modify your data on all websites.." [or whatever the exact wording is]. So, you're still letting the installed scripts have access to your browsing. One good thing about <whatever>Monkey and userscripts is that they make it trivially easy to inspect the scripts and see what exactly is being done. Whereas a browser extension is more of a 'black box'. So, there is more potential to spot potentially nefarious code in a userscript.js.