This is not FireFox that did this. FF is legitimate software. What you're describing is malware or scareware (once they ask for money to fix something or to stop something, it will cross the line from malware to scareware.).
More likely, a site you went to was infected and avast failed to stop it. This could have happened with Internet explorer / Chrome or FF. unfortunately.
The general rule of stuff like this is if you didn't ask for the software, don't install or pay for it. Internet helper is NOT good software.
Pop up blocked may be from Firefox, but I don't think it is. I've never seen that. You still didn't let us know if the Ad Block plus was in that window.
No, it's not normal to have your PC run slowly because of FF. My FF does run slowly because I have sometimes 70 tabs open, but only FF runs slowly because of that, not the whole PC. The reason your computer is running slow is the malware that's running on it.
This is something that several sites online can help you remove whatever's on your machine - for free, don't ever pay for it. I could help you if I was closer. Do you have a family member who's good with computers? I wouldn't be shocked to find that it's Search Conduit that you have installed. It's incredibly invasive, and I think it goes without saying that it's "bad" as well.