EDITED: Unfortunately, I have discovered a few more issues:
1. When right clicking a link and telling it to open in a new group, the new group will open, but the tab will be blank. That's a pretty annoying issue.
2. When using the Tree Style Tab and Tab Mix Plus addons and there is a problem with startup/restart, the new tab '+' button that comes after your open tabs and the close tab buttons you can place on the tabs themselves (TMP option) will very often disappear and Firefox will need to be restarted again to make them reappear. I realize this might not be the fault of this addon, but it is worth mentioning.
To be clear, I really like this addon; it works fairly well, and I would definitely recommend it to others who like organization. However, I personally have run into 2 issues that bring down the rating for me.
1. After using Firefox for a long time, like when the memory usage is super high and such, I can get several errors ("out of memory", "error code ###", etc) upon trying to restart - it also asks me to save a backup of the groups each time this happens so I can end up having to close 4-5 popups before Firefox will close -, and sometimes, tabs will move to the main group if something goes wrong at startup. This isn't a "for certain every time" kind of thing though, but it is often enough to be annoying for me since Firefox often needs to be restarted to fix memory issues and such. (I can provide screenshots).
2. The button to close a group on the TabGroups toolbar sits directly over the button to close a tab on the Tabs toolbar when tabs are set to 'on bottom' and using TabMixPlus' close tab button (a setup many people use). It's just SOOO darn easy to hit one when you mean the other! Thankfully, you can recover closed groups, but still... being able to customize the close group button's location to the other side or maybe hide it entirely should be added.
Other than those two things, keep up the great work!