Not only that, it is worrying that developers do not put Firefox testing into their development. I am sure the more sites they do the harder it is going to be to get them all fixed.
Firefox is undoubtedly the best web browser on the market at the moment and it will continue to grow especially with the support of Google.
Realestate.com.au works fine with the Firefox, but Domain.com.au still has some issues – although I understand they have fixed up most of the problems in the past few months. I understand these companies have massive development budgets so there should be no excuse but I doubt it would take less than 3 hours per site for web developers to get their sites up to speed.
So I suggest you download the Firefox browser here and test your website to see if it works fine. If your site is not working accordingly then contact your developer immediately to get this fixed. If you site was developed in the last 12 months I think it should be their responsibility otherwise you might find you will have to pay for it.