Possible discrepancies of ad impressions caused by Firefox 52

This article contains detailed information about possible discrepancies of ad impressions generated by Firefox 52 between 2017-03-07 to 2017-03-16.
At 2017-03-15 the Cxense Display team become aware about an unusual increase of impressions generated by Firefox on a website served by one of our customers.
Initial investigation revealed that this started slowly on 2017-03-07, with the release of Firefox 52 - and gradually increased. This turned out to be caused by a bug in Firefox 52's prefetching of pages. The bug itself was later reported here:
The bug manifests itself in such way that while a users visited page – it also reloads ad-requests for previous page-views made on the same domain.
All production system was patched in order to detect and discard impressions generated by this bug at 2017-03-16 09:00 (UTC). Since then all such requests returns HTTP error-code 400 - and are not counted.
Initially we considered this site an isolated case. But later it has turned out, using our browser-statistics, there there might be other customers affected by this bug as well. (Although the vast majority of customers should have not been affected)
For this reason Cxense will not invoice ad-impressions generated by Firefox 52 between 2017-03-07 and 2017-03-16.

What this means for you

For the few customers actually affected:

Cxense will identify customers with placements affected by this during the first week of April – and offer a down-adjustment of their impression data in the system for this period - that corresponds to the surplus of Firefox-traffic during the period. This however will have the following drawbacks:

  • Already finished campaigns might show up as under-delivering.
  • Not all reporting-data can be updated in this way (targeting-parameters such as geo/category/name-value, etc). This will create somewhat conflicting reports when generating them in Cxense Display.
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