This one: http://www.boomeranggmail.com/ ?

Is it completely removed, or is it getting disabled? Disabled extensions appear at the bottom of the Add-ons page. Either:

• Ctrl+Shift+a
• "3-bar" menu button (or Tools menu) > Add-ons

In the left column, click Extensions. Then check the bottom of the list on the right. Is Boomerang there, but disabled? If so, is there any explanation for it being disabled?

Boomerang was there but disabled. Unfortunately, I removed it and the explanation thinking I could reinstall to fix problem. I think it had something to do with Boomerang not meeting Foxfire's security requirements. Sorry I can't be more specific.

Have you tried reinstalling the current version fresh from their website? It's possible that older versions were not signed, which is a requirement in Firefox 43 and higher. See: Add-on signing in Firefox.

I tried that but can't find the Boomerang add-on there. Don't see it on the page and it doesn't come up when I use Boomerang as a search term.

Oh... it's not on Mozilla's Add-ons site, you need to go to their site:

http://www.boomeranggmail.com/

I get a huge red button that says "Add this to your Gmail" and when I click that, it leads to this page:

http://www.boomeranggmail.com/download.html?autoinstall=False

Firefox then displays an approval panel from the "control center" icon to the left of the address in the address bar.

If you do not see the red button, do you see "INSTALL" at the end of the page's header bar?

Yes, I've installed it from the Boomerang site and used the approval panel to the left of the address bar to stop Firefox from blocking installation. It installs and works for a day or two but then uninstalls again.

I've been in touch with Boomerang and changed my update options from default to automatic, as they suggested. Hopefully that will work. Boomerang is working since the last time I installed it today. We'll see what happens! Thanks so much for your help.