custom mode not work

There is 0 reason that an extension which manipulates the User-Agent HTTP request header needs "Access your data for all websites". It should not need ANY access to request or response content. This is a high risk and I would discourage people from using it for this reason. Your sensitive information, including usernames and passwords could be at risk of compromise.

helpful and informative

I had an addong called useragent switcher before and this aint it! Wish I could find it. I don't know what to do in this current version. I used the button on the tool menu and selected a newer version of Firefox but when I click test it still shows my actual version--52.5 ESR and it doesn't fool the addons site when I try to install a new addon. So far none of the user agent addons are working, so adding to the list of fails.

Can you spoof UserAgent from header ?
Check on https://browserspy.dk/useragent.php
Not working on first visit.