I just have a link to cynwoody version but I’m getting a 503 Server not found now.
Nope. Try to follow that link.
https://route66custom.com/frpfe@cynwoody.appspot.com.xpi
Come to find out, those xpi files sit in your firefox profile's extensions folder so anyone who has the addon installed, has the xpi file for installing it.
Your FF profile should be backed up on occasion. To see your profile(s), put this in for a url - about:profiles
Your profile folder holds extensions, settings, history, bookmarks, even currently open tabs.
More;
https://support.mozilla.org/en-US/kb/back-and-restore-information-firefox-profiles
https://support.mozilla.org/en-US/kb/profile-manager-create-remove-switch-firefox-profiles
Chrome only allows installing extensions from the Chrome store now. The file extensions are also different. CRX for Chrome vs XPI for Firefox. Not sure what the file contents differences are.