Sorry, but 0.5.2.0a is only a beta version.
Please try USI in Version 0.5.1.12a, this version works on my Android 5.0.2 Device with Firefox 58.0.2 .
If this Version, also didn't work for you, please give me more informations for debugging the problem.

Edit 1:
First, can you check if USI is even enabled?
Maybe you disabled it.
Second, reset all USI settings in the configuration of it.
Third, load the example userscript. It should generate a window alert message if you click on a page. For example try it on Google.com.

Which version did you use?
If you use Firefox 57, please try to install USI 0.5.1.x.

And if you find a bug, please create an issue at bitbucket.org

Thank you