Sorry, but that is up to Avira to fix. They need to supply their users with extensions that are signed by Mozilla, otherwise without an extension being "signed" it will be disabled in Firefox.

There has been at least 10 months notice about the need for all extension developers to get their extensions signed by Mozilla. For the extensions hosted by Mozilla they were signed en masse months ago, and new or updated extensions submitted for hosting at the AMO website are automatically signed as part of the screening process. Self-hosted need to be submitted and once the signing process is completed they will be sent back to the person who submitted it with the necessary "signature".

The need for signing was pushed-back a few times from Firefox 40 to Firefox 43. I suspect that Avira isn't too good at meeting deadlines and thus leaving their customers "in the lurch" now.

My advice is to contact Avira support for help with this issue.

I see. Thank you very much for answering in such a timely manner. I have contacted Avira about the problem, as have several others.

Thank you for this info. I have been having trouble with Avast freezing Thunderbird and if it continues will have to uninstall it. Avira has an excellent program and would be my first choice to replace it. I hope they fix this extension problem very soon. I don't want to replace freezing in Thunderbird with other problems in Firefox. I hope someone will post here if it's fixed. Thanks.