Review for Private Tab by Cakehole
Rated 5 out of 5 stars
One idea, change the colour of private tabs so they're easier to distinguish from normal tabs (I know the purple mask icon is there but I sometimes forget about it)
Or just change the favicon to the purple mask
See https://github.com/Infocatcher/Private_Tab#styles
To create your own collections, you must have a Mozilla Add-ons account.