I ran into the same thing for Firefox 43 when Mozilla changed from putting a warning about "unverified" add-ons to blocking them.  Here is the workaround:

(1) In a new tab in Firefox, type or paste "about:config" (no quote marks...) in the address bar and press Enter/Return. Click the button promising to be careful.
(2) In the search box above the list, type or paste "xpi" (again, without quotes) and pause while the list is filtered
(3) If the "xpinstall.signatures.required" preference is bolded and user set to true, double-click it to switch it to false.

This worked for me with regards to several add-ons that would no longer function and allowed me to manually enable them.

The real fix is for the developers (hello, IBM) to get their add-ons "verified" by Mozilla so that it isn't necessary to disable the signature requirement.