Firefox should not do that. It certainly does not do that for me and I am sure we would be aware if a lot of people saw such an issue. It is a rather unusual problem.

Do you have security or backup software that could be doing that I wonder. or maybe malware or an unauthorised version of Firefox.? You may need to check for malware but first of all lets backup the profile and do a clean reinsatall of an official Firefox.

Follow these instructions to backup your profile. That is where Firefox stores all your settings.

• Back up and restore information in Firefox profiles

Also  please 

• Make a careful note of the Firefox profile path and the full name of the profile.
  Note that is the profile that we would expect Firefox to use after the clean reinstall.
• State briefly what settings you expect to find are reset, and afterwards what settings are reset.
• Please do NOT use any Refresh option that you may come across.

Now do a clean reinstall, that involves deleting the existing program files (NOT the profile) The current version you install will be Version 48, you have posted from Firefox 47 so that will be an update of Firefox full instructions lower down.

Please post back with how you get on. If for some unknown reason you lose settings again you will need to look at

• Recovering important data from an old profile

Clean ReInstall

This means you remove your Firefox program files and then reinstall Firefox. This process does not remove your Firefox profile data (such as bookmarks and passwords), since that information is stored in a different location.

To do a clean reinstall of Firefox, please follow these steps: Note: You might want to print these steps or view them in another browser.

1. Download the latest Desktop version of Firefox from mozilla.org (or choose the download for your operating system and language from this page) and save the setup file to your computer.
2. After the download finishes, close all Firefox windows (or open the Firefox menu and click the close button ).
3. Delete the Firefox installation folder, which is located in one of these locations, by default:
   • Windows:
     • C:\Program Files\Mozilla Firefox
     • C:\Program Files (x86)\Mozilla Firefox
   • Mac: Delete Firefox from the Applications folder.
   • Linux: If you installed Firefox with the distro-based package manager, you should use the same way to uninstall it - see Install Firefox on Linux. If you downloaded and installed the binary package from the Firefox download page, simply remove the folder firefox in your home directory.
4. Now, go ahead and reinstall Firefox:
   1. Double-click the downloaded installation file and go through the steps of the installation wizard.
   2. Once the wizard is finished, choose to directly open Firefox after clicking the Finish button.

More information about reinstalling Firefox can be found here.

WARNING: Do not use a third party uninstaller as part of this process. Doing so could permanently delete your Firefox profile data, including but not limited to, extensions, cache, cookies, bookmarks, personal settings and saved passwords. These cannot be easily recovered unless they have been backed up to an external device! See Back up and restore information in Firefox profiles.

I followed the instructions. When trying to delete the Mozilla Firefox folder, I received an access denied even though my account is the administrator. I couldn't find any evidence of any Firefox-related services running, but I'm not an expert on such things. I rebooted and was able to delete the folder. The re-install appears to have fixed the problem. Hopefully, it won't reappear at the next update.

As for the question about possible malware, I run Avast Internet Security and Malwarebytes Pro all the time and haven't seen anything suspicious.

Based on my old geezer memory, I think the problem may have started when I upgraded Windows 7 Pro to Windows 10 about 6 weeks ago. I know there have been issues where some software needs to be re-installed after that upgrade. Perhaps Firefox falls into that category??