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.

There was an issue on Linux several months back that seemed to ignore some of the agent settings, but this appears to have been fixed. Thank you! Recommended, great extension.