Could you test whether Firefox can successfully download the extension file and save it to disk? Right-click the blue "Add to Firefox" button and Save Link As to save the file. Does it save successfully?

(Note: .xpi files are .zip archives, in case you wanted to look inside.)

That's a negative - Shows download attempt failed.

Hmm, thank you for testing that.

Extension downloads redirect from one server to another. For example, this is the first and second links for uBlock Origin. Do either work for you?

• https://addons.mozilla.org/firefox/downloads/file/3293994/ublock_origin-1.20.2-an+fx.xpi?src=dp-btn-primary

• https://addons.cdn.mozilla.net/user-media/addons/607454/ublock_origin-1.20.2-an+fx.xpi?filehash=sha256%3A4634411118e2598f91dbb07289b11109e097660d64391cd04eceb205d2c50854

Some users seem to have difficulty accessing servers whose names include CDN (a common abbreviation for content distribution network). Can you download the file in any of your other browsers?

Both work, and now so does everything else. It seems the problem solved itself. Could this have been a temporary issue with the source servers?

The server was working for me, but perhaps because it is a content distribution network, we are hitting different regional data centers.

Firefox is still doing this. This is very frustratrating. I have two addons out of the three that I use that will not synchronize because of this issue. Is there a workaround?

Chuck H said

Firefox is still doing this.

Hi Chuck H, your Firefox can't install any new extensions because Firefox thinks they are all corrupt? Is it the current version, i.e., not a version from May 2019 or earlier?

Did you try the download step to see whether the problem is occurring there: right-click the Install button the extension's page on the Add-ons site, and use Save Link As.

I will try that. I'm sorry that I didn't mention that this happened in Fedora Games and does not seem to happen as much in other Linux installs I have or in Windows so much. They all seem to reject Xtreme Download Manager and Eversync initially while allowing Lastpass to install but they will sync to Firefox eventually on other OSes except for Fedora.

I am able to download the addons directly but the problem may still between with Fedora as it seems.

Sorry, please ignore the earlier issue. I was able to update Firefox and now it all works fine.