Hi phasegen, what do you mean by this:

I have an add-on that I manually install.

How are you installing it? For example, on the Add-ons page, clicking the gear button and choosing "Install Add-on from File..."?

Firefox does a signature check once a day and disables extensions that can't be verified. The install location can also matter or links to specific web pages in its manifest file.

I install from a link on a website, though I do have the option to download it and do a manual install. Is there a setting for Firefox to notify me of a problem or update without correcting it? I really don't want to uninstall Firefox.

Okay, so whether you install using a link on a website or locally, Firefox is able to verify and enable the extension. But then at some later time, the extension is not being disabled for reasons such as not being able to be verified, it is completely disappearing from Firefox?

Yes. It is being uninstalled. Completely. Off the list. Twice now in three days.

I can't think of a Firefox feature that would do that if the extension passed all the usual checks at install time less than a day before.

Do you want to set the extension .xpi file to read-only and see whether that has any effect? It would be in the profile folder under /extensions. It could be difficult to identify by name, so you might open the internal about:debugging page so you'll be able to look up its its GUID.

Then see: Profiles - Where Firefox stores your bookmarks, passwords and other user data.

Does this happen when Firefox is running or when you close and restart Firefox?

What extension is this about?

You can possibly check the extensions.json file in the profile folder to check its install data. The Browser Console should also have messages when there is an extension check done.

When I close and restart. The extension is Dissenter.

I've marked it read-only and it has stayed put with no errors