Does other addons install besides these? Problem addon are not a ff browser end user help to fix but must be directed to the developer to find out if there is anything wrong when installing it.

I also tried the following, randomly selected, add-ons https://addons.mozilla.org/en-US/firefox/addon/groupspeeddial/?src=collection https://addons.mozilla.org/en-US/firefox/addon/default-bookmark-folder/?src=collection https://addons.mozilla.org/en-US/firefox/addon/webext-private-bookmarks/?src=collection https://addons.mozilla.org/en-US/firefox/addon/onenote-clipper/?src=collection

I think it is safe to say it is not a problem with a specific add-on.

I just tested this on a Windows 10 machine and did not have any issues.

I tested a passwords manager and https://addons.mozilla.org/en-US/firefox/addon/onenote-clipper/?src=collection

Are you still seeing this happen?

WestEnd said

Problem addon are not a ff browser end user help to fix but must be directed to the developer to find out if there is anything wrong when installing it.

This is a community forum of users helping users of things related to (desktop) Firefox, including with Extensions.

Yes, some may get a better answer from the developer of the extension on complicated issues but it is not a requirement for people to go elsewhere for help with Extensions not made by Mozilla.

Not all third-party extension authors or companies that made the Extension does one on one support so you cannot expect Firefox users to rely on only that option for help. Many of these extensions are made available by the author for other people to use and may have little free time from work and life to be doing one on one support. I know one author of a particular popular extension quit making it (passed it on to other people) because it took too much in his busy schedule not only in making/maintaining it but with people expecting him to give them one on one support on every little issues.

Besides the OP was not necessarily having a issue about any particular Extensions but just with installing the Extensions (like the various examples listed in posts) from addons.mozilla.org/firefox/ for some reason.

Also issues with the addons.mozilla.org site can be posted about in https://discourse.mozilla.org/c/add-ons

Can you save the XPI file to your desktop (right-click: Save Link As) and install the file locally via the extension manager?

It is possible that security software (anti-virus, firewall) is causing the problem. Try to disable security software temporarily to see if that makes a difference.

Got frustrated with it, so I downloaded Firefox 60.4.0esr and installed it. I no longer having the Add-on install issue. I cannot tell you (with absolute certainty) what my Firefox version was before upgrading to 64.0.2 (64-bit) because my upgrade history (in about:support page) is empty, which I am assuming got erased when performing some troubleshooting steps from various articles. I believe it was 59.02. I can say this because I "froze" my previous version because I had crashing problems caused by Digital Gaurdian (which is installed by my IT group and not allowed to disable or remove it).

https://support.mozilla.org/en-US/questions/1204196

I am suspecting that this download issue may also be related.