hello, do you have norton as your av provider by chance?

I do have Norton.

then the likely source of the issue is that norton is FALSELY recognising an integral component of firefox as malicious during the update process and deleting it continually (people with the same error message have reported that in the past). there is not much that mozilla can do against such third-party influence...

please look into your norton's configuration to see if something can be done about this (maybe whitelisting the mozilla firefox folder) or otherwise use a more reliable and unerring security vendor.

I'm a user of the product; it isn't my place to solve MOZILLA's issues, whether they be caused by NORTON or MICROSOFT.

The product doesn't work and "if" the problem is caused by a major player such as NORTON, its the responsibility of MOZILLA to resolve the conflicts.

Otherwise, there shouldn't be surprise of losing customers.

This sometimes happens with the upgrade stubs.

Download Firefox For All languages And Systems v43* {web link}