Folder tabs start 'active' instead of unloaded (maybe bug? maybe FF?)

[dsl101]

I use TST as a kind of bookmark system for often-used tabs, and have created 'folders' using the extension group tab as the URL, e.g.:

moz-extension://1847b511-80b4-436b-a403-d34ff09050e9/resources/group-tab.html?title=Wordpress

for a load of wordpress-related pages. I keep these tabs 'unloaded', and have an extension which allows you to use these unloaded group tabs as full-width expanders (opening / closing their trees wherever they are clicked).

This has been working brilliantly until this week, when suddenly FF is starting up with all those group tabs 'active' instead of unloaded. I've no idea if this is a change in TST or FF, or how to find any solution to it—it's totally messed up my workflow. Does anyone know if this is now just a feature, or if it can be configured?

[dsl101]

Just for reference, these are my related browser settings, which haven't changed recently: