FirstAid plugin worked on QGIS 3.10/3.16 on my Windows 10 machine until a couple of weeks ago. I had to modify my PYTHONPATH environment variable for another purpose and the plugin no longer breaks either for my plugin code or standalone script.
I am using Windows 10.0.19043 and have tried re-installing the plugin, installing QGIS 3.10 standalone and updating QGIS to 3.18 via OSGEO4W but nothing has resolved the problem so far.