I have a Galaxy S10. Data saver is on. Auto update is off. Backup is off. There are no cloud apps or cloud access. I don't stream music or videos. Before putting the phone to sleep, I exit all open apps. I installed the firefox app last week. Due to the holidays, the app was rarely used last week. On Friday, I used it to check the weather. I closed the tab before exiting Firefox. At about 2 AM, while I was asleep, I started getting text messages from my carrier, Verizon. The first told me I had reached my 4GB data limit. That was followed by 5 others in rapid succession adding additional data. I was up to an additional charge of $100 before my sleep fuzzy brain kicked into gear and I turned wifi on. The Verizon website said the data was used for Cloud/File Transfer. I don't have cloud acess on this phone and I did not initiate a file transfer. The phone itself said that all the data was used by Firefox operting in the background. Even though data saver was supposed to prevent apps from running in the background, apparently Firefox carved out an exception for itself when it was installed. You have to drill down through the network menu on the phone to find this. Needless to say, I uninstalled Firefox.

I have used Firefox for years and would like to use it on my phone, but I can't handle surprise phone bills. And I would like to know what what Firefox was doing that used up 5GB of data in two minutes. If Verizon was correct and data was transfered, what was transferred and where did it go? Since I did not give permission for this or initate it, I find it very distrubing.