Please navigate to Settings - Supported browsers and check if Firefox is listed as "Installed". If it´s not installed, install it by pressing the "Install" button. If it´s installed, uninstall it by pressing the "Uninstall" button and then reinstall it by pressing the "Install" button".
It said "Invalid", then I clicked uninstall after which it said "Not installed", then I clicked Install and it gave the error in my previous post again and the status became Invalid again.
When I tried again at work it had the same error again. I think it's an issue with the portable version only. At home I installed the non-portable version (to update the version) and it worked. Then I used the portable version to install the firefox addon at work and the error occured again.
I did that, but it didn't work. I solved it by installing sticky password to my work pc too and installing the firefox addon from there. Then the portable version worked again... (also after uninstalling sticky password from the work pc again)