Out of interest, are you on the main fork of Firefox or the ESR version?
Me: Seriously i don't know what a ESR version is!

Latest Fiurefox version is 64.0 and I recommend the 64bit version. You can check your version here: >Menu (3 lines)>Help (2nd last)>About Firefox. If you have something blocking JavaScript, you need to unblock that for gog.com.
Me: I believe i downloaded the 64Bit vesion. It's around a month since i bought this PC at a mall at Black Friday. Seems logic i downloaded that 1. Could also be that it was set up when i got the PC. I will check this.
No need to answer with questions since i fast loose the ability to give feedback

Neither. Currently I'm using Brave.

So, uh, maybe post this in the original thread that you created:

https://www.gog.com/forum/general/addon_to_updates_ruins_logins

Would seem to make more sense then starting a third thread about the matter.

By a lucky shoot i was able to answer now! I checked and i use Firefox 64.0
It might seems funny but the prob. is true.
went in and checked my firefox and i close the case.
No way i can fix this anyway.

So the conclution is GOG don't work for me. As i mentioned earlier
Steam works well

Event tried to download GOG Galaxy 1 more time but when trying to install i met the message:
Already installed or a newer version in use!

Tnx and a happy new year
Special tnx to those trying to help

I promise that everything would have been in 1 discussion if i could.
As i said my possibellity to answer dissapear....