Cool little module if you need to set a maximum/particular number of tabs to be loaded/made active on startup, but this will require that value to be set to 'false' in 'about:config' in order to work properly:

browser.sessionstore.restore_tabs_lazily

I use it in conjunction with Sidebery to have my last 5 to 10 tabs remain active to visually sort between them, so it's a nice workaround to the "all or nothing" standard Firefox behaviour that would either make all tabs active or just the last one.

It's super niche but I'm glad I found it, thanks to the author!

PS: only problem is that the tabs appear to be made/become inactive only when switching to another tab