Each time I start Firefox I kick off between 7 and 10 tabs by middle-clicking a bookmark folder. It now seems to take ages until any of the tabs finish loading. It never used to take this long. I'll see if your suggestion makes any difference, thanks.
EDIT: Didn't make any noticeable difference, alas. FWIW: PaleMoon is like lightning in comparison. Also IE based browsers are a lot faster.