A temporary workaround:
Navigate to about
:debugging
Click the 'Load Temporary Add-on' button
Navigate to the .xpi file in your profile directory select it and click Open.
To find your profile directory and extensions:
Windows:
C:\Users\{your username}\AppData\Roaming\Mozilla\Firefox\Profiles\{profile id}.default\extensions
Linux:
~/.mozilla/firefox/{profile id}.default/extensions/
MacOS:
~/Library/Application Support/Firefox/Profiles/{profile id}.default/extensions
Or just download the extensions you want from
https://addons.mozilla.org/:
Right click on the 'Add to Firefox' button
Choose 'Save Link As' from the menu
Save the .xpi file somewhere handy
Navigate to about
:debugging
Click the 'Load Temporary Add-on' button
Navigate to the .xpi file you downloaded and open it
Restarting Firefox will remove any temporarily loaded extensions.