I just wanted to share this in case it helps or saves others. I have been working in Nigeria for a week. My office gave me an Etisalat 3G 'easyblaze' wifi device for Internet access. A few days into my stay, I started receiving multiple popups in Firefox wen accessing some web sites. The popups were all blocked by Firefox. Web sites such as BBC News and CNN.com were particularly affected by this. Clicking on white space on the page would also bring up assorted advertising pages. Secure web sites were not affected. Also, my backup browser Opera was not affected. I naturally assumed my PC had some how been infected. this despite running up to date McAfee which had detected nothing. I installed MBAM and a few other utilities which detected nothing. So I tried a clean install of Firefox which made no difference.
After a lot of experimentation, I found that clearing the Firefox cache (Settings/Advanced) would provide a temporary fix. However, after a while the adware problems reappeared in Firefox - though MBAM at east blocked them effectively. The only new software I had installed was Teamviewer, which I doubt would be any issue.
It occurred to me that maybe my computer was not infected at all. I reallied that Opera was most likely not affected because its Turbo feature was enabled. I guess that would at least protect all traffic as far as Operas servers. So I now started wondering about the Etisalat device and their network. That was easy to verify by using the browser on my phone which was also connected to the same wifi device. Sure enough, using the Opera browser on the phone (no Turbo) the adware problem appeared. To me, that points pretty clearly to Etisalat's system being the source. To further verify this, I used my laptop on my work network, having cleared the cache and no problems appeared.
A i said, I am posting this in the hope it will help others. I am perfectly prepared for the experts here to disagree with what I have found! My final comments are 1. Don't forget to clear your cache, 2. If you have to use Etisalat Nigeria, use Opera with the Turbo feature on.