Possible, but I think the more likely explanation is that Firefox users are just higher risk. I don't think the slightly increasesed rare of captchas is going to drive people off of Firefox.
No, but in whatever little bot/scraper blocking work I did, user-agent blocking was an incredibly powerful low-hanging fruit.
For sensitive material, we would often insta-ban anyone who hit us with a 'curl' user-agent and that definitely killed a lot of script-kiddies' dreams right then and there.
Obviously, if you are a determined "hacker" you would get past that and hit the honeypots but simple filters could filter out enough scrapers such that the rest of the attacks could be manually evaluated by humans.
