For SignatureSwitch, see the developer's comments:

http://mozext.achimonline.de/about.php

Quicktext is an option.

What are plans to allow them to work again?

This is a question for the authors of those add-ons... If you can't live w/o some add-ons then you might downgrade to v68 till those add-ons (or similar ones) will be compatible with v78. The downgrade option is not recommended though...

Version 78 is incompatible with all add-ons I've been using. In this configuration the version is not very usable. I lost connection to google contacts and calendar. Sorry Thunderbird is not an island. There should be a strategy add-on developer can keep up with Thunderbird development. Reading their comments I am no longer sure, that Thunderbird is any longer my first choice.

I have the same problem with Quicktext so Quicktext doesn't appear to be an option after all! It is loaded and enabled but the list of signatures doesn't appear, so I can't put my professional info on my emails anymore. Very boring. I've signed up so you can let me know when this is fixed. I've put a screenshot up so you can see - nothing happens when I press the Quicktext button and there is no longer a signatures pull-down menu below the 'paragraph' box' where there used to be. Jane

Re last reply, all is well. I closed Thunderbird and opened it again and Quicktext now works! Not sure why. I enabled Signature Switch as well - was that it?

Jane