Hi German.
You have debugged the issue here correctly. There is a problem with concatenated strings:
$string['firefox_warning'] = 'There are some problems with Firefox at the moment. ' .
'Please use a Chromium based browser like Chrome, Brave or Opera for a better experience.';
This problem is not visible at Github as clearly the maintainers no longer publish their recent development there! Their repository was last updated 7 months ago. I am going to report this as an issue -- plugins are expected to have their Git repository updated even after they are approved.
For the record, the developers are informed about troubles with AMOS integration of their plugin. This is how their Dev zone page looks for them:
