Very strange Bug / Thin Fonts

Hello and congratulations for plugin: it works fine but I noticed a strange bug ??

Before installing the fonts they looked good and readable, that is like this (https://imgur.com/MaEWtiA) instead after the installation they look like this (https://imgur.com/0HQZEDY), thin, almost transparent and bad!

However, this is only on my pc’s Firefox browser; not on chrome, not from mobile, nor on other computers with firefox. I tried uninstalling firefox and disabling all browser components but the problem remains.

And if I disable the plugin, everything looks good again on my browser: what could it be due to? it is very important for me since I develop sites! Thank you very much