when you get that it means that firefox is re-doing an action that you have already done. If you are searching for something this is generally fine, but the warning is there so that you don't, say... register for two accounts on a website, or send the same contact form twice, or worse, buy the same thing from an online shop multiple times without meaning to. to avoid getting it, you can sometimes click on the url bar at the top of the page and hit enter instead of refreshing. Personally, as a web developer, I wouldn't recommend trying to turn it off (I'm not sure if it's even possible, to be honest).