Try installing the extension Maximize All Windows (on Startup).

Does it still happen in a new profile (without signing into sync account)? An easy way to test a new profile is to install Developer Edition and see if it happens there or refresh your existing profile.

Thank you for the answer zeroknight. I will explore that. I am a little averse to having to fiddle with my browser constantly though. I just want one that works when I need it.

Try to rename/remove xulstore.json in the Firefox profile folder with Firefox closed.

You can use the button on the "Help -> More Troubleshooting Information" (about:support) page to go to the current Firefox profile folder or use the about:profiles page (Root directory).

• Help -> More Troubleshooting Information -> Profile Folder/Directory:
  Windows: Open Folder; Linux: Open Directory; Mac: Show in Finder
• https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data

Thank you cor-el. I have done that a few times, and it works for a while. We'll see if it "sticks" this time. Last time it took a few days to exhibit the behavior again, so not sure what it going on to make it recur.

What security software do you have in case that is interfering ?

I have AVG. I have it installed on several other laptops, and have used it for many years. I will however compare the settings to this particular laptop.

So far the last deletion of xulstore.json seems to have resolved the issue. Thanks for all suggestions.