When Kaspersky gets their add-ons signed by Mozilla. https://support.mozilla.org/en-US/kb/add-on-signing-in-firefox

well it is time for Firefox to get their backsides into gear-this is much too slow!

Mozilla has been prepared for "signing" for over 3 months now. [And the Add-ons Blog announced this program all the way back in Feb 2015, for the Firefox 40 version; that "deadline" was pushed back to 43 due to lax add-on developers waiting until the last minute.] Mid-summer all the Add-ons that are hosted at the Add-ons website, which were compatible with "signing", were "signed" automatically. But Add-ons that are included with other programs [like Kaspersky], or available from 3rd party websites need to be submitted to Mozilla Add-ons for "signing"; after which they would be returned to the person who submitted it.

I suspect that Kaspersky waited too long to submit their Firefox add-on/s and now they might be in the queue for the "signing" process. OR maybe Kaspersky hasn't submitted their add-on/s yet?

That said ....

There is an override that you can enable.

https://support.mozilla.org/en-US/kb/add-on-signing-in-firefox#w_what-can-i-do-if-firefox-disables-an-installed-unsigned-add-on

Override add-on signing (advanced users): You can override this setting by changing the xpinstall.signatures.required preference to false in the Firefox Configuration Editor (about:config typed in the URL bar). Support is not available for any changes made with the Configuration Editor so please do this at your own risk.

Note: That pref will be gone in Firefox 44 (Release slated for Jan 26. 2016) so user's of currently un-signed extensions should contact the developers of those extensions and prod them into getting their act together & get their extensions signed by Mozilla. Otherwise the users of those extensions will be SOL come the release of Firefox 44. Mozilla has already pushed back the version in which this new security feature was to be enabled from Firefox 40 to 44. I doubt if there will be any further reprieve.