About media.net , I gave it a try last year. It was good the two first weeks, then collapsed to near zero. I never understood why. They made me sign at something at Google, which they claim will grant access to a premium inventory of advertisers for my site. But it didn't change anything. Text ads could have been interesting, but, when you had 5 links, this was 5 variants of the same sentence, which I guess will not be appealing to visitors. And, when you were clicking on one of them, it was taking forever to reach a page, with more links, which were taking themselves forever to load the advertiser's site. The only positive thing at Media.net, is that it's easy to discuss with an account manager. Mine, always answered in a mater of minutes to my emails.
Now, since media.net is contextual advertising, if you insert their code into a blank page, to be used as adsense's backup ads, there will be no "context" to match for the ads. So I assume they might have a global theme for a site, which can be used, I don't know.
About ads not showing on specific browsers. In my case, it was still loading my backup ads. I could tell it was not only from my computer, by looking at the number of impressions of my backup ads page. And during the 2 days I had this problem, roughly 30% of my visitors saw my backup ads, instead of adsense. Then last night, all returned to the "normal", with nearly no impression of my backup ads. I have no idea what happened, since it worked with Chrome (and nothing related to being logged to Google account, or add-ons installed, because I always clear all the cache / cookies when testing). So this was a "glitch".
Now, all is normal, and my adsense earning is in the upper range.
Also, since always, I often see errors message in the console logs of Firefox (or Chrome), coming from third party networks ads. I don't know how it impacts or not earnings, since ads seem to show anyhow.