If you are using newer version of a Firefox it will not allow to install the said addons. From the time Firefox update to 43 and newer that there is some security features being added into it. I would suggest to use an old version to get it and successful installation from your end. But, there is an option in order to accept it but its is quite not secure and it will void the security if you made some modification in the setup of firefox. I believe somewhere here also, mentioned here the steps for it.

Robert Mendoza

On 4/16/2016 6:40 PM, Ali Colak wrote:

Added to my previous message, I've encountered this problem
previously and it isn't something rescent. Previously I just asked
someone sited to click the button for me.

On 4/16/16, Ali Colak <alish54@...> wrote:

Or am I doing something wrong.
Hello to all. Last night I tried to reinstall webvisum, as I have
just baught a new computer. When ever I press the install link, the
notifaction to allow firefox to install the addon comes. I've tried
pressing alt+a several times, and I've tried pressing the space bar or
enter when it seems that I'm on the allow button, nothing works.
I don't remember seeing any other complaints about this from anyone,
so is it only me getting something so basic wrong?

Any suggestions?
Ali