Check your zoom controls.

<Control> (Mac=<Command>) + (plus) enlarge - (minus) reduce 0 (zero) restore

Is this with all websites/pages, some, a few?

Start Firefox using Troubleshoot(Safe) Mode {web link}

A small dialog should appear. Click Start In Troubleshoot(Safe) Mode (not Refresh). Is the problem still there?

We're not talking about screen scaling. We're talking about the scroll bar disappearing. And yes, the problem is the same in Troubleshoot Mode. Actually, the same thing happens in other browsers, so maybe it's simply by design.

BTW, the zoom control is OPTION-COMMAND +or-.

Many site issues can be caused by corrupt cookies or cache.

Warning ! ! This will log you out of sites you're logged in to. You may also lose any settings for that website.

• Delete cookies to remove the information websites have stored on your computer {web link}

• How to clear the Firefox cache {web link}

OK, we're not making any progress here. As I said, this happens in other browsers, so if it's a cookie problem, then all browsers somehow have that problem. That doesn't make sense.

Hi Dan, there's an OS-level setting for scroll bar hiding:

https://support.apple.com/guide/mac-help/change-general-preferences-mchlp1225/10.15/mac/10.15

Yes, BINGO! I realized this while you were typing, jscher. I see the scroll bar disappearing for EVERY application I use. So it isn't an application issue, it's a system issue. As you point out, it's really easy to fix. Thanks.