The add-on could not be downloaded because of a connection failure – Firefox error
If you cannot download and install add-ons and you receive an error Tadd-on could not be downloaded due to connection failure On your Firefox browser, this is how you can still download and install it. The error occurs because the server is not responding to the user request.
Add-on could not be downloaded due to connection failure
1) Manually download the extension
Just before we start, since we are downloading a file, I suggest you use a download manager. In case of problems, you can resume the download.
Open the add-on page and find the Add to Firefox button. If you are using a third party website, look for the download link.
Right click on the Download link or the Add to Firefox button. To select Save as.
It will be saved as .xpi file. You can choose the default name or add your name.
Press the Enter button to start the download.
When the download is complete, if prompted, choose to open it with Firefox. The addon will be installed. Otherwise, you can choose to open with Firefox.
Go to about: config then click on "I'll be careful, I promise" button.
Type xpinstall.enabled to find the parameters
Double click to set the value to true
While these two should be enough to download the extension in case of connection failure, sometimes you have to wait. It may be a temporary problem that is resolved on its own. In the end, I will recommend using VPN software. If there is something your ISP is blocking, the VPN connection should be able to resolve.