Have you tried a re captcha yet? They're awful on firefox (read: non-chrome), especially if not signed into a google account.
https://news.ycombinator.com/item?id=20058697
And the back and forth on whether you really need it:
I don't use Chrome, so I can't tell how bad it is on a comparative basis, but (especially in private browsing mode) Recaptcha can make you cycle through a half-dozen different images looking for the traffic lights or what not before letting you continue.
That's when I just leave. If I'm feeling nice, I'll drop the site owner a note explaining why, but usually not.
Nobody's site is that great.