I've been using this extension for a long time, and I could not live without it. I highly recommend id.

Howerver, I would love to have keyboard shortcuts for accessing & sorting containers (CTRL+period doesn't work on my azerty OS X keyboard). This is a real usability problem for me, it's so annoying to use the mouse to open a tab...

Merging and splitting windows based on tabs would be nice too (merge all windows, split all "Google" container tabs into new window, etc).

Another annoying thing is opening content from other apps. Firefox should ask for a container to open the link in.