how do I block "ads not by this site" from appearing ?
Ads appear on pages being viewed eg. MSN. How can these be blocked since they generally appear over text so it's not readable.
You have malware/adware. Reset Firefox or scan with MSE, Hitman Pro or other anti-malware capable program. Ads Not by this site is not that harmless, as it collects both info about sites you visit and displays advertisements.
What you need to do is remove an extension called Service x86. There are of course more such extensions, a short list is available here:
However, I'm pretty sure it's the Service x86 because I didn't see any other extension than this in a past few days. This one is being distributed rather actively. By the way, they all use the same pattern so maybe it would be a good idea for FF to block the whole "family" of these extensions.
And one more thing, if you won't remove the culprit, I mean adware that installed this extension, it will probably come back even if you reset FF. You may use a free tool called AdwCleaner to remove adware from your computer. Of course, you can use any other if you wish.