Works only for first request to page???

The fact that you can't edit a header to fix an issue makes this plugin a pain to use. And .... you can't even select it to copy and paste has made me remove the plugin.

Remove & modify both doesn't work
Once a rule added can't be edited
Unfortunately

i really needed multi User-Agent switcher like this.
but not work in FF 54.
any other condition required for use this?