It means that site was hacked and someone installed JS scripts that try to hack users PCs
or
admin of that site loads some .js (jquery, ADs systems etc.) files from other sites and one of them was hacked and tried to install virus on all sites that load file from it.
Now that domain is on list of 'unsafe sites' that uses Firefox, so all Firefox users will see that warning.
Google, Firefox and many other firms scan internet and publish lists of 'unsafe sites'.
My friend site was on google list, after he fix everything he had to wait 5 weeks before they removed it from list [it's automatic, so you cannot do anything]. It took so long, because his site was not popular (5-10 users per day).