All of a sudden it is now working. I cannot tell you how or why, just kept at it. Very weird. So no longer an issue for me.

Try updating to FF ver 54 current version and see if that changes anything. From FF Menu Bar: Help then About If solves your issue please come back and mark as solved.

I've marked your above answer as the solution since you wrote that your problem was fixed.

Note that this happens if the extension's XPI file needs to be unpacked because it want to modify files or otherwise needs to access files in the XPI file. Firefox normally doesn't unpack XPI files, there need to be an unpack directive in the install.rdf file of the extension.

Dashlane will not install in my W7 laptop running FF56 64 bit. In place of Dashlane all I get is a blank page. I have all ad blockers etc turned off.

I got locked out of Lastpass a few weeks ago, mebbe I should just stick o FF!

Nevertheles I'd appreciate any help

Thanks

honeyhill