You can find all older versions (except legacy) on the versions page:

https://addons.mozilla.org/en-US/firefox/addon/all-tabs-helper/versions/

If you want the last legacy version, you can install a signed version from here:

http://kevinallasso.org/extensions/alltabshelper/legacy_0.2.38/[email protected]

(If you do this, I would suggest first _removing_ the Quantum version of ATH and restarting Firefox)

You can also download it zipped from here:

http://kevinallasso.org/extensions/alltabshelper/legacy_0.2.38/[email protected]

EDIT: In answer to your last question, as I said I am unable to duplicate the issue. If you want to see if there is a particular setting that triggers it, you can reset all options, and toggle them one by one to see if and when this becomes an issue.

EDIT 2: Can you contact me at [email protected]?