There is an issue with the configuration upon upgrade. If there is a proxy configured without a username/password, then both fields become the string undefined upon upgrade. For some reason, this doesn't fully take effect until the first time the configuration is saved--at which point the use of the proxy stops working.

Steps to reproduce:

1. Start with a fresh firefox profile.
2. Install FoxyProxy 7.5.1 from https://addons.mozilla.org/en-US/firefox/addon/foxyproxy-standard/versions/?utm_content=search&utm_medium=referral&utm_source=addons.mozilla.org
3. Import a proxy: socks5://localhost:8801?title=01&proxyDns=false
4. Upgrade to FoxyProxy 8.7 (go to about:addons, then click the gear, then click "check for updates" -- firefox will update FoxyProxy automatically by default).
5. Enable the proxy and test browsing; everything should still work ok.
6. Within the FoxyProxy settings, configure the proxy. Note that the "username" and "password" fields are set to the string undefined.
7. Click the "Save" button.
8. Test browsing; firefox is now "Unable to connect".

Thanks,
Corey