Just adding a picture as it wasn't uploaded.

If I try to close the dialogue box, another one opens up very quickly with no chance of me clicking anything else. Eventually it opens full screen and I get a message saying that "Firefox prevented this site from installing any software". I also do not get the option of preventing further dialogue boxes.

I have FF 57 I think.

Please use more than 1 scanner as each uses diff tech : https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-caused-malware Save your Report and google each before deleting anything as do not want to delete something you need, If need help : https://forums.malwarebytes.com/topic/9573-im-infected-what-do-i-do-now/ Post in only 1 forum, then wait.

Alternatively if are free from malware report the site for this as is abuse.

Please let us know if this solved your issue or if need further assistance.