A quick search shows that you have already posted about a similar issue around a month ago:
The team members’ responses lead me to believe that everything is alright, whatever is happening appears to be only on your end and by the looks of it it sounds like the hosts you use are modifying the add-on itself, which invalidates the signature.
Try downloading the signed add-on from AMO to your desktop and install it through the Firefox Add-on manager page:
If it installs correctly and it is no longer disabled after browser restarts then it is confirmed that something is modifying the add-on when you host it yourself.
Never modify a signed add-on, once you change anything inside a signed add-on its signature will become automatically invalid.