Please explain the problem in detail. What happens? What is the exact error messages?

What is the link of the add-on you want to download?

Hello Refugio,

Stab in the dark, but looking at your user agent (that's shown in your system details) I'm thinking that that just might be the problem.

Would you please take a look at this article and see if that will make a difference :

https://support.mozilla.org/en-US/kb/how-reset-default-user-agent-firefox

Apart from your user agent being anonymized - it also shows that you need to update Firefox to the latest version, which is 64.0.2

If that's not it, then maybe you'd like to go over this article :

https://support.mozilla.org/en-US/kb/unable-install-add-ons-extensions-or-themes

Does any of this help  ?

hi, sorry have been on the road, but the ptoblem still remain i do have the latest firefox version i cleaned the cache, restart etc. but on the addon download site i'am always ask first download firefox as it is not on my system well, thanks a lot for your kind advices -think about going to a computer service and try to fix it with theire help

Did you fix the User Agent string as McCoy mentioned?

User Agent: Mozilla/5.0 (Windows NT 10.0; rv:63.0) Gecko/20100101 Firefox/63.0 anonymized by Abelssoft 1623918525

That extra added string misidentifies your Firefox version.

yes, finaly it works thanks a lot super support