One of the greatest things about Firefox is that it is an endlessly extendable browser. Your web browser can be about far more than simply browsing the internet, and there are all manner of add-ons available that can add all sorts of features to enhance existing options or provide you with completely new utilities. But the sheer number of add-ons that exist, coupled with the constant updates to Firefox, means that there are going to be problems with some add-ons that become incompatible as browser development continues.
Not all browser add-ons are updated as frequently as Firefox itself, and it may be the case that the developer is not aware of any problems. Add-ons Compatibility Reporter enables you to send a report to Mozilla letting them know that you have experience compatibility problems with an extension – the more people do this, the greater the chance that developers with be able to fix problems in future releases.
Recent updates to Firefox means that this is not as useful as it used to be, but it is still handy to have installed.