Funny, we get more complaints about DuckDuckGo browser than anything else, and that's one of the few we don't test on. I know this because I make it a point to have someone from CS tell me about consistent pain points users are having. I wonder how many complaints about Firefox not working your customer service team is getting daily and you just don't hear about it because they've been told to tell users "just say Firefox isn't a supported browser and to try installing Chrome."
You should ask someone in CS. Whichever agent bullshits the least (not the manager) - you might learn something.
Almost 3/10 people accessing your sites are using Firefox. All those "images not loading right or whatever" are probably blatant to them, making them think "wow, what an absolute shit website."