niarha said

With the new version of Firefox, I can't install the add-ons or themes. [I have an] error and tells me to check the network connection, but everything works, except for the Firefox installations.

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con la nuova versione di firefox non riesco ad istallare i componenti aggiuntivi nè temi, da errore e mi dice di controllare la connessione di rete, ma funziona tutto, tranne appunto le istallazioni firefox

There was a Bug-A-Boo with a Certificate Expiration Date dealing with the Add-ons.

See Mozilla Blog Post.

Try to install this HotFix Extension:

https://storage.googleapis.com/moz-fx-normandy-prod-addons/extensions/hotfix-update-xpi-intermediate%40mozilla.com-1.0.2-signed.xpi

~Pj

If you haven't installed the 'HotFix' Extension yet: New FF version 66.0.4 is out:

There was a Bug-A-Boo with a Certificate Expiration Date dealing with the Add-ons. There's a new FF Version with the main Fix, 66.0.4.

Mozilla Blog Post Update:
A Firefox release has been pushed — version 66.0.4 on Desktop and Android, and version 60.6.2 for ESR. This release repairs the certificate chain to re-enable web extensions, themes, search engines, and language packs that had been disabled (Bug 1549061).
There are remaining issues that we are actively working to resolve, but we wanted to get this fix out before Monday to lessen the impact of disabled add-ons before the start of the week. More information about the remaining issues can be found by clicking on the links to the release notes above.
(May 5, 16:25 EDT)

~Pj