

It’s very unlikely you are infected by anything unless you were using some crazy settings or addons, or unless you were hit by some extreme 0-day exploit that hasn’t become widespread yet. Firefox does not and normally cannot execute files it downloads automatically nor are videos a likely risk for remote code execution now that we have technologies like data execution prevention built into processors, if you’re attacked by malware it will rely on some other vector or trickery to get you to execute the file. I would expect that your performance issues are unrelated, but you should also check Firefox’s addons and extensions as well as your task manager startup tab to make sure nothing has obviously been installed without your knowledge.
One thing that sticks out at me is the fact that you only mention the file’s “title” and if you haven’t already you should make sure Windows Explorer is set up to ALWAYS show full file extensions, that’s like a basic safety measure that really should be on by default but isn’t, and it’s really mandatory if you’re messing around on the darker parts of the web. You have to know what kind of file extension it is because that affects what Windows is going to do with it, and when it’s supposed to be one thing and Windows is going to do something different with it that’s a huge red flag that it’s malware trying to trick you into running it.
You can upload the file to virustotal if you want to scan it but it doesn’t sound likely that it even ran unless you did something bad by accident.





My old LG SmartTV seems most reliable at playing mkv files but I think mp4 is pretty standard.