If the devices are managed with Android+ (DA) then this script can be used to clear the Android App Cache. Keep in mind that App Cache != Browser Cache but it will definitely clear the Browser Cache as part of that process. If they are AE, I don't think the wipeapplication script is working. 

wipeapplication org.mozilla.firefox

As in any application in the background? Or just one application? Why do you need to kill applications in the background?

@Matt there are some inhouse application that are being used by one web application.  in house application are running in background. some time they are misbehaving so our L1 team has to kill them manually.