You don't even need to hover over an advert in the browser* to be infected in some cases, the mere loading of the ad can be enough when certain exploit methods are used. my advice would be to run an ad blocker, a script blocker and an anti-exploit program. Noscript for firefox is a good scriptblocker, malwarebytes anti-exploit is a good anti-exploit program, both are free. The ad blocker will directly block adverts, the script blocker will block adverts as a side effect but primarily it will act to prevent the loading of anything which would make exploits possible, the anti-exploit program sits as a layer of defence behind these but infront of your antivirus and catches anything that looks to be trying to break free from the browser and take control of the rest of the system.
One other tip, make sure no plugins are set to be automatic, disable those you don't use, set the rest to "ask to activate"/"click to play" this will help further prevent video adverts and certain types of exploit, many rely on plugins but a few rely on the browser itself, which is why you want a script blocker.
*adverts in other programs are a whole lot more confusing, whether they can endanger you depends on the exact nature of the program they are displaying in.
Edited by rp88, 22 February 2016 - 12:12 PM.
Back on this site, for a while anyway, been so busy the last year.
My systems:2 laptops, intel i3 processors, windows 8.1 installed on the hard-drive and linux mint 17.3 MATE installed to USB