If the app is being uninstalled by itself, there's a possibility you may have malware on the device or specific settings on the device to wipe 3rd party apps. You can sign up for a Firefox Account to sync history and add-ons across devices.. So, the next time you log in if you must reinstall again, your add-ons will be there.

I don't think neither of the reasons you have mentioned is the case. Also I don't think syncing add-ons is possible as of yet!I already have a sync account! The problem is more prevalent when the phone is switched off or terminates unexpectedly.

No app erases is by itself, nor will Firefox delete itself, this is along the lines of more of a hardware and device software issue and not a application issue as the app is not built to delete upon restarts. In most cases, when a app deletes itself, it may be because the phone is low on memory space and there's a setting for it to delete the least important application, the device is experiencing some sort of software malfunction, glitch or bug (in which, the Android version you're running), or there may actually be some hardware issues with the device in which that is complicated to resolve There are apps like Lookout Security which can scan your phone for you, which is what I would recommend you do rather than telling me my advice is not invalid. Is entire application deleted when you go to look for it or is an icon still on the device? After the application is installed, can you check if the app is using the storage on the device or of an external sd card. More so, is " move to 'x' " greyed or available to be pressed on, you can try experimenting with this and see if it makes a difference with reboots. If in all ends the above fails or does not meet your expectation, I would suggesting resetting your device. Warning: you assume all liability and risks associated with resetting your device. You also are aware that resetting the device will cause it to lose all it's data in which you may want to backup your device before doing a reset. You are correct that add-ons for sync is not available on Android, and I do apologize - rather it is available on Desktop instead. You could store a draft online somewhere or write down what add-ons you have installed so you have a list available, relying on us to keep a record isn't entirely impossible, we it is hard to meet the needs of every user. That being said, you can provide suggestions and feedback using the following link... https://input.mozilla.org/feedback

Thank you

I would like to apologize if I sounded rude or offensive.

I totally agree with what you said (just eliminating the possibilities)

No other app (probably and rarely mx player pro) does the same as Firefox does. The problem, I think, might be due to low memory space (but I am not sure about it). How can I prevent that? Can I move Firefox to my sd card( placing it in the system memory as well as sd card poses a problem).? Also when Firefox is gone, I mean completely gone, no icons, no leftovers. (My devices runs on Android 6.0)