You should contact that developer to see if they will make one for Firefox. Otherwise you have to search the Firefox Addon page for such Addons if one exists.

Thanks, I'll contact the developer. I looked around on the addon page but did see anything.

You mean, something like this? https://askubuntu.com/questions/853891/how-can-i-make-firefox-run-in-the-background-like-chromium

Thanks, that might work but it looks quite involved. The one in Chrome is a simple extension so I was hoping for something like that. The developer of Lightening Reopen said an extension or addon like it isn't possible in Firefox. Maybe I'll just leave Firefox open all the time.