What is "firefox for Acer"; please don't answer "I assumer..."
Firefox will not update, but continuaaly load unstoppable popups suggesting I download and install a new copy. This is "Firefox for Acer". One other question about this was answered "I assume that you can ask this at the Acer support forum as they should know this best." wgich means, I suppose, that the responder doesn't knowm but likes answering questions.
Does anyone know? Is this why Firefox fails to update? Will re-installing eliminate the problem? Is there any way to stop the popups?
Firefox is open source software and companies can decide to make modifications and brand their unofficial Firefox version like Acre does with its "Firefox for Acer version. Only Acer knows what modifications this is about, so we can merely suggest to switch to Firefox from the Mozilla server in case of unexplained issues to see whether that helps.
If there is a problem with updating then can consider to switch to Firefox from the Mozilla server. You can find the full installer of the current Firefox release (92.0) in all languages and all operating systems here:
Firefox might refuse to keep using your profile and wants to create a new profile. Firefox 67 and newer use a dedicated profile for each Firefox installation and lock this profile to prevent other Firefox installations from using this profile.