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Is it open source?

Ta opinia jest o wcześniejszej wersji dodatku (3.7). 

Hello,
Sadly no, none of my addons are open source yet.
I'm working on my addons full-time and I'm trying to make it a working business which seems to be easier with closed source.
But if it helps, I'm official Friend of Addons :)
https://blog.mozilla.org/addons/2020/06/15/friend-of-add-ons-juraj-masiar/

Best regards,
Juraj Mäsiar