Review for Extra Folder Columns by lethe
Rated 5 out of 5 stars
Looks like this is abandonware.
Edit: Thanks for the update.
Hi, this isn't abandonware, it just hasn't needed to be updated for any changes. It runs nicely as it is.
Unfortunately, it looks like there was a bug or issue somewhere, that caused the maximum compatible version to not be automatically updated on addons.mozilla.org, I've now manually.
To create your own collections, you must have a Mozilla Add-ons account.