In some cases, the HP UFT Testing Agent can no longer be enabled via the Add-ons Manager in Firefox 44.0 due to a lapse in its add-on signature. Until HP addresses this, you can temporarily override add-on signing in order to enable the HP UFT Testing Agent extension again.
(reference: https://support.mozilla.org/en-US/kb/add-on-signing-in-firefox?as=u&utm_source=inproduct#w_override-add-on-signing-advanced-users)
1) In Firefox, navigate to about:config (Firefox Configuration Editor), then click through the warning page to continue.
2) Search for xpinstall.signatures.required and double-click to set to false.
3) You now have the option to Enable the HP UFT Testing Agent extension in Add-ons Manager.
4) Restart Firefox and verify the extension is Enabled in Add-ons Manager.
5) Once HP updates the extension's signature, be sure to change xpinstall.signatures.required back to true in the Firefox Configuration Editor.