How to install unsigned add-ons in Firefox 43. Mozilla has turned on extension signing enforcement by default in Firefox 43, means to install an extension it needs to be signed and verified by Mozilla, users who’ve upgraded from Firefox 42 to version 43 noticed some of their extensions were disabled mentioning the reason as unsigned.
But still you can enable those in this version, and but not in the next version 44, as browser company has provided override only for this version.
Re-Enable disabled unsigned extensions in Firefox 43
1. Visit about:config
2. Find preference xpinstall.signatures.required and change its value to false by double clicking on it.
Or rather than you do that manually, install this add-on Install unsigned add-ons (link), which toggles the above mentioned preference value.
For the next version, if your using extension is not-signed by then, then you need to search for alternative add-ons on AMO and install them, or ask developer to submit his add-on to Mozilla for signing , if the developer has already dropped support with the commencement of Web Extensions API, then you can’t do much.