That has nothing to do with Firefox since firefox doesn't make such request. You need to clear the cache/history to remove this if this is in the cache loading. Remember do so will clear all login and history but your login should remain but if not you should write or store them down somewhere safe before doing this.

I delete the site and then it comes up om every other site I go to and I need to delete it again

You need to scan your computer for malware.