When I clicked on firefox's full screen popup to install firefox's 51.0.1 upgrade a second small popup representing itself as mozilla firefox said that I should install something called EasyPDFCombine by myway on firefox as my home page. I first ignored this second popup and just clicked on the firefox install button to install the upgrade for firefox, and in so doing nothing happened; i.e., this EasyPDFCombine popup appears to have blocked or prevented the upgrade installation. I then went back to the EasyPDFCombine popup and clicked on it to OK its installation and then again I clicked on firefox's install button; which at this time then the upgrade appeared to install. Which at the time appeared to be OK. Later I discovered I could not, that is, was not allowed to change my homepage to firefox. Moreover, I discovered that the EasyPDFCombine program was not even listed on my computer so that it could not even be uninstalled. It appears that EasyPDFCombine is clearly a form of malware operating under the guise of Mozilla's Firefox program. This therefore is definitely a matter that Mozilla and Firefox must concern itself and look into. PS: My operating system is Windows 7.