Foxit has known vulnerabilities, but then do does the Adobe Reader, and if Adobe can't get it right.......
When something like this happens you have to ask a few questions
exactly what is giving the warning?
what is the software doing at the time the warning happened?
whats the real risk?
Foxit has ALWAYS downloaded toolbars and junk software when installed. It also has a nasty habit of offering these at random as an "upgrade". My guess is thats whats happened here. If you simply stick to downloading and installing the main Foxit reader program and deselect the addons, you should be OK
And Foxit doesn't come from the Mozilla add-ons page. It comes from the Foxit website......