I don't know that it's firefox. I think it's possibly a pup.conduit thing. I'd be sure I had the latest firefox, yes because one version a time or two back DID slow things down, but they sent out a "update to stop this" thing a while back.
But when my thing starts slowing at all, I find that in my research, I've landed on a site that has attached that dumb pup thing to me and malwarebytes just takes it away again. Easy peasy. no more sluggies.